Method and electrochemical cell for managing electrochemical reactions

ABSTRACT

A method and/or electrochemical cell for utilising one or more gas diffusion 5 electrodes (GDEs) in an electrochemical cell, the one or more gas diffusion electrodes have a wetting pressure and/or a bubble point exceeding 0.2 bar. The one or more gas diffusion electrodes can be subjected to a pressure differential between a liquid side and a gas side. A pressure on the liquid side of the GDE over the gas side does not exceed the wetting pressure of the GDE during 10 operation (in cases where a liquid electrolyte side has higher pressure), and/or a pressure on the gas side of the GDE over the liquid side, does not exceeds the bubble point of the GDE (in cases where the gas side has the higher pressure).

TECHNICAL FIELD

The present invention relates to the management of electrochemicalreactions that involve the simultaneous presence of liquids and gases,as reactants or products. Example embodiments relate to methods, cellsor devices that facilitate or assist in managing such electrochemicalreactions.

BACKGROUND

Numerous electrochemical processes involve gas-to-liquid orliquid-to-gas transformations. For example, hydrogen-oxygen fuel cellstypically utilize the transformation of gaseous oxygen and hydrogen intoliquid water at solid-phase, electrically-connected catalysts, likeplatinum metal.

Many gas-to-liquid or liquid-to-gas processes are most effectivelycarried out by so-called Gas Diffusion Electrodes (GDEs). At the presenttime, commercially available GDEs typically comprise fused, porouslayers of conductive particles (usually carbon particles) of differentsize. The outer-most layers typically contain particles of the smallestdimensions, fused together with lesser amounts of hydrophobic PTFE(polytetrafluoroethylene, or Teflon™) binder. The inner-most layerstypically contain the largest particles. There may be multipleintermediate layers of intermediary particle size.

The intention of this gradation in particle size within GDEs, fromlargest in the center to smallest on the outer sides, is to create andcontrol a three-phase solid-liquid-gas boundary within the electrode.This boundary should have the largest possible surface area. Thecreation of such a boundary is achieved, effectively, by controlling theaverage pore sizes between the particles, ensuring that the smallestpore sizes are at the edges and the largest are in the center. Since thepores are typically relatively hydrophobic (due to the PTFE binder), thesmall pore sizes at the edges (e.g. 30 microns pore size) act to hinderand limit the ingress of liquid water into the GDE. That is, water canpenetrate only a relatively short distance into the GDE, where theelectrochemically active surface area per unit volume, is largest. Bycontrast, the larger pores in the centre of the GDE (e.g. 150 micronspore size), allow for ready gas transmission at low pressure along thelength of the GDE, with the gas then forming a three-waysolid-liquid-gas boundary with the liquid water at the edges of the GDE,where the electrochemically active surface area per unit volume is thelargest.

Layered porous electrode structures are presently the industry standardfor:

-   -   (1) conventional free-standing GDEs (for example, of the type        used in hydrogen-oxygen PEM fuel cells); and    -   (2) hybrid GDEs, where a GDE layer has been incorporated within        an electrode, typically between a current collector and the gas        zone.

GDEs of this type often display significant technical problems duringoperation. These largely derive from the difficulty of creating aseamlessly homogeneous particulate bed, with uniform pore sizes anddistributions, and uniform hydrophobicity (imparted by the hydrophobicPTFE binder within the GDE). Because of the resulting relative lack ofuniformity in the GDE structure, the three-phase solid-liquid-gasboundary created within the GDE may be:

-   -   Unstable and fluctuating. The location of the boundary within        the GDE may be subject to changing conditions during reaction        which cause the boundary to constantly re-distribute itself to        new locations within the GDE during operation.    -   Inhomogeneous. The boundary may be located at widely and        unpredictably divergent depths within the GDE as one traverses        the length of the GDE.    -   Inconsistent and ill-defined. At certain points within the GDE,        there may be multiple and not a single solid-liquid-gas        boundary.    -   Prone to failure. The boundary may fail at certain points within        the GDE during operation, causing a halt to the desired chemical        reaction. For example, a common failure mode is that the GDE        becomes completely filled with the liquid phase, thereby        destroying the three-phase boundary; this is known in the        industry as “flooding”. Flooding is a particular problem in fuel        cells, such as hydrogen-oxygen fuel cells, that require the        feedstock gases to be humidified. Flooding may be caused by        water ingress into the gas diffusion electrode via systematic,        incremental percolation through the non-homogeneous pores of the        electrode, or it may be caused by spontaneous condensation of        the water vapour in the feedstock gas stream. In all cases,        flooding induces a decline in the voltage output and power        generation of such fuel cells.

Problems of this type are not conducive to optimum operations and mayresult in uneven, low-yielding, incomplete or incorrect reactions,amongst others.

Conventional 3D Particulate Fixed-Bed Electrodes and GDEs

At the present time, 3D particulate fixed bed electrodes and gasdiffusion electrodes (GDEs) are conventionally fabricated by mixingcarbon black and PTFE powders and then compressing the solid mixtureinto a bulk, porous electrode.

The pore size of the resulting structure may be very roughly controlledby managing the particle size of the particulates used. However, it isdifficult to achieve a uniform pore size throughout the electrode usingthis approach because particles, especially “sticky” particles likePTFE, often do not flow evenly and distribute themselves uniformly whencompressed. A wide range of pore sizes are therefore typically obtained.It is, moreover, generally not possible to create structures withuniformly small pore sizes, such as 0.05 μm-0.5 μm in size.

The hydrophobicity of the structure is typically controlled by managingthe relative quantity of PTFE incorporated into the structure. The PTFEholds the structure together and creates the required porosity. However,its quantity must be carefully controlled so as to impart the electrodewith an appropriately intermediate hydrophobicity. An intermediatehydrophobicity is needed to ensure partial, but not complete wateringress. In the case of GDEs, this is needed to thereby create asolid-liquid-gas boundary within the carbon black matrix that makes upthe electrode.

This method of constructing 3D particulate fixed bed electrodes and gasdiffusion electrodes creates some significant practical problems whenoperating such electrodes in industrial electrochemical cells,particularly in electro-synthetic and electro-energy (e.g. fuel cell)applications. These problems include the formation of three-waysolid-liquid-gas boundaries that are: ill-defined, inconsistent,unstable, fluctuating, inhomogeneous, and prone to failures likeflooding.

Problems of this type largely arise from the intrinsic lack of controlin the fabrication process, which attempts to create all of the inherentproperties of the electrode—including porosity, hydrophobicity, andconductivity—in a single step. Moreover, the fabrication method seeks tosimultaneously optimise all of these properties within a singlestructure. This is often not practically possible since the propertiesare inter-related, meaning that optimising one may degrade another.

Despite these drawbacks, the approach of combining particulate carbonblack and PTFE into a compressed or sintered fixed bed remains thestandard method of fabricating GDEs for industrial electrochemistry.This approach is used to fabricate, for example, free-standing GDEs ofthe type used in hydrogen-oxygen PEM fuel cells. Even where only a GDEcomponent is required within an electrode, the standard method offabricating that GDE component is to form it as a compressed, porouslayer of particulate carbon black and PTFE.

For the above and other reasons, the conventional method of making GDEsand the properties of conventional GDEs, and methods for managingelectrochemical reactions occurring therein, are open to improvement.

FIG. 1 (prior art) depicts in a schematic form, a conventional 3Dparticulate fixed bed electrode or a gas diffusion electrode (GDE) 110,as widely used in industry at present.

In a conventional 3D particulate fixed bed electrode or GDE 110, aconductive element (e.g. carbon particles) is typically combined (usingcompression/sintering) with a non-conductive, hydrophobic element (e.g.polytetrafluoroethylene (PTFE) Teflon™ particles) and catalyst into asingle, fixed-bed structure 110. The fixed-bed structure 110 hasintermediate hydrophobicity, good but not the best availableconductivity, and a pore structure that is non-uniform and poorlydefined over a single region 113. When the 3D particulate fixed bedelectrode or GDE 110 is then contacted on one side by a liquidelectrolyte and on the other side by a gaseous substance, these physicalfeatures bring about the formation of an irregularly-distributedthree-phase solid-liquid-gas boundary within the body of the electrode110, below its outer surface 112 and within single region 113, asillustrated in the magnified view presented in FIG. 1. At thethree-phase boundary, electrically connected catalyst (solid phase) isin simultaneous contact with the reactants (in either the liquid or thegas phase) and the products (in the other one of the liquid or gasphase). The solid-liquid-gas boundary within the GDE 110 thereforeprovides a boundary at which electrochemical liquid-to-gas orgas-to-liquid reactions may be facilitated by, for example, theapplication of a particular electrical voltage. The macroscopic width ofthe three-phase solid-liquid-gas boundary is comparable or similar indimension to the width of the conventional GDE. The thickness of thethree-phase solid-liquid-gas boundary in a conventional GDE is typicallyin the range of from 0.4 mm to 0.8 mm in fuel cell GDEs up to, higherthicknesses, such as several millimeters, in industrial electrochemicalGDEs.

Overpressure and Flooding

The phenomenon of flooding described above is often caused by ingress ofliquid electrolyte, for example water, into the gas diffusion electrodewhen the liquid electrolyte is subject to any sort of external pressure.For example, in an industrial electrolytic cell of 1 meter height, waterat the bottom of the cell is pressurised at 0.1 bar due to the hydraulichead of water. If a GDE were used at this depth, the GDE would typicallybe immediately flooded by water ingress because modern-day GDEs havevery low “wetting pressures” (also known as the “water entry pressure”),that are typically less than 0.1 bar (although GDEs with wettingpressures of 0.2 bar have recently been reported in WO2013037902). GDEsare, additionally, relatively expensive.

This is a particular problem in industrial electrochemical cells inwhich it is highly beneficial to apply a gas, for example such as oxygenor hydrogen, to an electrode, via the use of a GDE at that electrode.Many industrial electrochemical cells are large, employing waterelectrolyte with a depth greater than 1 meter in the cell. If a GDE withwetting pressure less than 0.1 bar is used, then the cell will leak fromits electrolyte chamber unless additional means are applied to balancethe pressure differential between the liquid and the gas side of theGDE.

Companies have therefore gone to great lengths to try to operate GDEs ator near 0.1 bar. For example, WO 2003035939 describes a method forsegmenting the water head so that a depth of more than 1 meter can beused but no part of the GDE feels more than 0.1 bar trans-electrodepressure.

In other industrial electrochemical cells, electrolyte is routinelypumped around the cell. Unless expensive pressure-compensation equipmentis installed to scrupulously avoid pressure differentials, such pumpingactions may readily generate local increases in the liquid pressure of0.1 bar or more, thereby causing the cell to leak from its electrolytechamber if a GDE was used as one of the electrodes.

The counterpart to the problem of flooding relates to the development ofexcess pressure on the gaseous side of conventional GDEs. If the gaspressure in a conventional GDE becomes even a little higher than theliquid pressure, then excess gas may pass through the GDE, exiting asbubbles at the liquid-facing side of the GDE. The formation of bubblesin this way is generally deleterious to the performance of the cell inthat bubbles typically: (i) “mask” the electrode surface, causing adecline in the rate of reaction, and (ii) increase the ionic resistanceof the electrolyte solution, inducing unnecessary energy consumption.

These problems related to pressure equalisation at the gas-liquidinterface in GDEs are compounded by the fact that many industrialelectrochemical cells operate most effectively when the liquidelectrolyte is pressurised to, for example, several bars of pressure. Asit can be extremely difficult to maintain pressure equalisation at aGDE, solid-state electrodes, such as dimensionally-stable electrodes,must instead be used. Such electrodes typically generate bubbles of theproduct gases, so that, the gas-liquid interface for such electrodesinvolves the surface of the gas bubbles present in the liquidelectrolyte at the electrode face. The gas within such bubbles mustnecessarily and automatically be at the same pressure as the surroundingliquid electrolyte, in order for the bubble to resist collapse, or toavoid uncontrolled expansion. Under these circumstances, pressureequalisation is not a challenge and the problem is thereby avoided.

The technical problems associated with commercially available GDEs,along with their high cost and other factors, mean that it is generallycommercially and technically unviable to use GDEs in many present-dayindustrial electrochemical gas-to-liquid or liquid-to-gas processes.

This is demonstrated by the very extensive efforts that have been madeover the years seeking to develop GDEs and pressure-equalisingapparatuses to deal with these challenges in the case of thechlor-alkali process. The chlor-alkali process is one of the most widelyused electrochemical processes in the world. Numerous patentapplications have described approaches seeking to overcome the problemof pressure equalisation in the chlor-alkali process. For example,Patent Publication Nos. WO 2003035939, WO 2003042430, and, morerecently, WO 2013037902, have described pressure-equalising apparatusesand/or fabrication techniques to create Gas Diffusion Electrodes able toavoid leaking up to 0.2 bar liquid overpressure.

In summary, in order to realise the benefits that GDEs may confer uponelectrochemical gas-to-liquid and liquid-to-gas processes, newelectrochemical cells, GDEs and/or methods to manage electrochemicalreactions or pressure differentials at the liquid-gas interface of GDEsare needed. Preferably, an improved Gas Diffusion Electrode should berelatively inexpensive, robust and/or mechanically strong, and have arelatively high wetting pressure.

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that the prior publication (or information derived from it)or known matter forms part of the common general knowledge in the fieldof endeavour to which this specification relates.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the Examples. ThisSummary is not intended to identify all of the key features or essentialfeatures of the claimed subject matter, nor is it intended to be used tolimit the scope of the claimed subject matter.

As used herein, we define two new quantities for Gas DiffusionElectrodes:

-   -   (a) “Wetting pressure” (or “water-inlet pressure”). “Wetting        pressure” is defined as the lowest excess of pressure on the        liquid electrolyte side of a GDE relative to the gas side of the        GDE, at which the liquid electrolyte penetrates and floods the        GDE. Wetting pressure (relative to water) is a well-known and        readily measured physical quantity that is widely used in the        field of porous membranes in the water treatment industry.        However, the concept has never been applied to GDEs.    -   (b) “Bubble point”. The “bubble point” is defined as the lowest        excess of pressure on the gas side of a GDE relative to the        liquid electrolyte side of the GDE, at which the gas blows        through the GDE and forms bubbles at the electrode surface on        the liquid electrolyte side in the electrochemical cell. “Bubble        point” is a well-known and readily measured physical quantity        (relative to water) that is widely used in the field of        hydrophobic membranes in the water treatment industry. However,        the concept has never been applied to GDEs.

Following from the inventors' realisations, at least some problemsreferred to in the preceding background section can now be redefined byreference to these terms. In effect, conventional GDEs have: (i) low“wetting pressures” (typically less than 0.1 bar), which causes them toflood (and leak) easily, and (ii) low “bubble points”, which result inready bubble-formation at the liquid side of the conventional GDE.

In one example aspect, there is provided a method for managingelectrochemical reactions. In another example aspect, there is providedan electrochemical cell for electrochemical reactions. The term“managing” is defined in this specification as: to proactively direct,arrange, contrive to arrange, accomplish, or exert control over anelectrochemical process in such a way as to bring about an industriallybeneficial outcome. In another example aspect, there are provided GasDiffusion Electrodes with improved, increased or high wetting pressuresand/or improved, increased or high bubble points. That is, example GDEswith a proclivity for flooding-free and/or bubble-free operation aredisclosed.

The inventors have found that example GDEs of this type or class canprovide inexpensive, robust, and/or mechanically strong electrodes thatcan have an unusually high electrochemical activity, and can,consequently, be readily, generally, and/or beneficially deployed as gasdiffusion electrodes in a variety of industrial electrochemicalprocesses, methods, cells and/or devices.

In one example form, there is provided a method for managing anelectrochemical reaction in an electrochemical cell, the electrochemicalcell having a gas diffusion electrode positioned between a liquidelectrolyte and a gas region, the method comprising: applying a pressuredifferential between the liquid electrolyte and the gas region that isless than the wetting pressure of the gas diffusion electrode; orapplying a pressure differential between the gas region and the liquidelectrolyte that is less than the bubble point of the gas diffusionelectrode; wherein these pressure differentials relate to the liquidelectrolyte that is produced or consumed and a gas that is produced orconsumed during operation of the cell; and wherein the wetting pressureor the bubble point is greater than 0.2 bar.

In another example form, there is provided an electrochemical cellcomprising: a gas diffusion electrode having a wetting pressure or abubble point greater than 0.2 bar relative to a liquid electrolyte and agas produced or consumed in operation of the cell; and the gas diffusionelectrode positioned between the liquid electrolyte and a gas region;wherein in use a pressure differential is applied between the liquidelectrolyte and the gas region that is less than the wetting pressure;or a pressure differential is applied between the gas region and theliquid electrolyte that is less than the bubble point.

In one example form, example 3D electrodes or GDEs of the currentembodiments are distinguished from conventional particulate fixed-bedGDEs in that they separate the key features of a 3D electrode or GDEinto two, or at least two, distinct regions, each of whose propertiesimprove upon and may be more fully controlled than is possible withinthe single body of a conventional GDE. An example embodiment of such a3D electrode or GDE may comprise a liquid-and-gas-porous conductivematerial, which can optionally also include a catalyst which is enhancedor optimized for its catalytic capabilities and conductivity. Theconductive material is attached to, coupled to, touching, positionedadjacent to, or abuts, a gas permeable material that is non-conductiveand liquid electrolyte impermeable during normal operational use of theelectrode, e.g. which may be hydrophobic, for which the pore structureis selected, enhanced or optimised for gas transport properties. Normaloperational use is, for example, when the electrode is functioning asintended and not flooded. In an example, a surface of the gas permeablematerial is facing the porous conductive material. The surface of thegas permeable material may, but need not necessarily, touch or contactthe porous conductive material, for example there may be an intermediarybinder material or layer that can include one or more catalysts. At ornear the surface of the gas permeable material is an interface orboundary region of the gas permeable material and the porous conductivematerial. When the electrode is in use, a three-phase solid-liquid-gasboundary is able to form at or near the surface of the gas permeablematerial facing the porous conductive material. In this context, “at ornear” the surface is intended to mean within a distance being thethickness of a binder material (if present, and as discussed herein), orwithin a distance being the macroscopic width of the three-phasesolid-liquid-gas boundary itself, or within a distance of any overlap ofthe gas permeable material and the porous conductive material, or withina distance being the width of the porous conductive material. Thethree-phase solid-liquid-gas boundary need not form precisely ‘at’ thesurface, but can form ‘near’ the surface in the sense of being close,neighboring, adjoining, immediately next to or within, or proximate. Thethree-phase solid-liquid-gas boundary can further move in response tothe application of an excess gas or liquid pressure, however theboundary will remain ‘near’ to the surface as described during normaloperational use.

Preferably, the two regions (being a first region including the porousconductive material and a second region including the non-conductive gaspermeable material) are substantially distinct, demarcated or separated,although they are positioned adjacent, abut, touch or adjoin each other,so that there is an interface or a boundary region, or possibly anoverlap.

In such an example embodiment, the non-conductive, liquid electrolyteimpermeable or hydrophobic, gas permeable material has pores that arebetter defined, more uniform, and of smaller average size, than can beachieved in a conventional GDE. The liquid-and-gas-porous conductor,preferably provided with a catalyst, may be more conductive than aconventional GDE, while its low hydrophobicity may see the porousconductor completely or substantially completely filled with liquidelectrolyte under normal operating conditions, thereby enhancing ormaximally facilitating catalysis. In contrast, in a preferred form, thehigh hydrophobicity of the non-conductive, hydrophobic, gas permeablematerial will typically see the gas permeable material completely emptyor substantially empty of liquid electrolyte at atmospheric pressure,thereby enhancing or maximally facilitating gas transport into and outof the GDE.

When such an example embodiment 3D electrode or GDE is contacted on theconductive side by a liquid electrolyte and on the non-conductive sideby a gaseous material, then the above physical features cause theformation of a three-phase solid-liquid-gas boundary at or near asurface of the gas permeable material facing the porous conductivematerial, which also can be at the interface between the two distinctregions. This boundary is quite different to the three-phasesolid-liquid-gas boundary in a conventional GDE. It differs in that itis better defined, narrower, more stable and/or more robust than can beachieved in a conventional GDE. Thus, in operation of a preferredembodiment, a three-phase solid-liquid-gas boundary forms at or near asurface of the gas permeable material facing the porous conductivematerial (which may also be at the interface, or a boundary region, ofthe porous conductive material, which can include a catalyst, and thenon-conductive gas permeable material). This provides a three-phasesolid-liquid-gas boundary with a relatively narrow macroscopic width,for example in comparison to the width or thickness of the electrode.

These features are important because the inventors have found thatexample embodiment 3D electrodes or GDEs can provide, at or near theinterface of the two regions, an enhanced or optimum pore structure, forexample hydrophobic pore structure, that facilitates improved or maximumgas transport, with an enhanced or optimally conductive, improved ormaximally catalytic structure. In effect, at the three-phasesolid-liquid-gas boundary in example embodiment 3D electrodes or GDEs,each of the critical properties of a gas diffusion electrode may be madeideal, or, at least, nearer to ideal than is otherwise possible.

The inventors have further found that the effect of this enhancement oroptimisation yields surprising and remarkable electrochemicalperformance. Despite the three-phase solid-liquid-gas boundary beingnarrower and confined to what appears to be a two dimensional (2D), orsubstantially 2D, macroscopic geometry, the electrochemical capabilitiesof the three-phase solid-liquid-gas boundary in example embodiment 3Delectrodes or GDEs substantially improves upon and, in fact, far exceedthose of conventional GDEs.

These enhancements provide unexpected improvements over conventionalGDEs. They appear to arise because the fabrication of conventionalparticulate fixed-bed GDEs as currently employed in the art, ispredicated on creating all of the important physical properties at thesame time within a single material. Such an approach effectively ignoresthe fact that the key properties of GDEs (namely: pore structure,hydrophobicity, gas transport, liquid transport, conductivity andcatalytic activity) are typically inter-dependent and are therefore notopen to ready, concurrent enhancement or optimisation within a singlematerial. Example embodiment GDEs as described herein take account ofthis limitation and separately optimise one or more of the keyproperties, to thereby achieve more ideal overall properties at theinterface of the two distinct regions.

As used herein, a three-dimensional (3D) electrode is a solid, gaspermeable or liquid flow-through electrode whose effective surface areais greater than the geometric 2D surface area of the electrode. 3Delectrodes are non-planar electrodes that typically improve thetransport of one or more reactant species to the 3D electrode's surface(by utilising the increased effective surface area). Reference to 3Delectrodes should be read as also including flow-through electrodes orporous electrodes.

Reference to a gas permeable material should be read as a generalreference including any form or type of gas permeable medium, article,layer, membrane, barrier, matrix, element or structure, or combinationthereof.

Reference to a gas permeable material should also be read as includingany medium, article, layer, membrane, barrier, matrix, element orstructure that is penetrable to allow movement, transfer, penetration ortransport of one or more gases through or across at least part of thematerial, medium, article, layer, membrane, barrier, matrix, element orstructure (i.e. the gas permeable material). That is, a substance ofwhich the gas permeable material is made may or may not be gas permeableitself, but the material, medium, article, layer, membrane, barrier,matrix, element or structure formed or made of, or at least partiallyformed or made of, the substance is gas permeable. The gas permeablematerial may be porous, may be a composite of at least one non-porousmaterial and one porous material, or may be completely non-porous. Thegas permeable material can also be referred to as a “breathable”material. By way of clarifying example only, without imposing anylimitation, an example of a gas permeable material is a porous matrix,and an example of a substance from which the gas permeable material ismade or formed is PTFE.

Reference to a porous conductive material should be read as includingany medium, article, layer, membrane, barrier, matrix, element orstructure that is penetrable to allow movement, transfer, penetration ortransport of one or more gases and/or liquids through or across at leastpart of the material, medium, article, layer, membrane, barrier, matrix,element or structure (i.e. the porous conductive material). That is, asubstance of which the porous conductive material is made may or may notbe gas and/or liquid permeable itself, but the material, medium,article, layer, membrane, barrier, matrix, element or structure formedor made of, or at least partially formed or made of, the substance isgas and/or liquid permeable. The porous conductive material may be acomposite material, for example composed of more than one type ofconductive material, metallic material, or of a conductive or metallicmaterial(s) and non-metallic material(s). By way of clarifying examplesonly, without imposing any limitation, examples of porous conductivematerials include porous or permeable metals, conductors, meshes, grids,lattices, cloths, woven or non-woven structures, webs or perforatedsheets. The porous conductive material may also be a material that has“metal-like” properties of conduction. For example, a porous carboncloth may be considered a porous conductive material since itsconductive properties are similar to those of a metal.

The porous conductive material may be a composite material, for examplecomposed of more than one type of conductive material, metallicmaterial, or of a conductive or metallic material(s) and non-metallicmaterial(s). Furthermore, the porous conductive material may be one ormore metallic materials coated onto at least part of the gas permeablematerial, for example sputter coated, or coated or deposited onto atleast part of a separate gas permeable material that is used inassociation with the gas permeable material. By way of clarifyingexamples only, without imposing any limitation, examples of porousconductive materials include porous or permeable metals, conductors,meshes, grids, lattices, cloths, woven or non-woven structures, webs orperforated sheets. The porous conductive material may be a separatematerial/layer attached to the gas permeable material, or may be formedon and/or as part of the gas permeable material (e.g. by coating ordeposition). The porous conductive material may also be a material thathas “metal-like” properties of conduction. For example, a porous carboncloth may be considered a ‘porous conductive material’ since itsconductive properties are similar to those of a metal.

A desirable feature of example GDEs of the current embodiments is theirability to contain electrolytes, for example water, acid, or caustic,within electrochemical cells and devices even at relatively high appliedpressures on the liquid electrolyte, whilst simultaneously bringinggases, for example oxygen or hydrogen, to the electrode interfacewithout any need for bubble formation or substantial bubble formation.Moreover, example GDEs of the current embodiments may be significantlyless expensive than conventional GDEs.

In a further example aspect, there is provided a gas permeable 3Delectrode comprising: a gas permeable material; and a porous conductivematerial attached to or positioned adjacent to the gas permeablematerial. In a preferred aspect, the gas permeable material isnon-conductive and liquid electrolyte impermeable, e.g. hydrophobic,during normal operational use of the electrode. Preferably, athree-phase solid-liquid-gas boundary is able to form at or near asurface of the gas permeable material facing the porous conductivematerial. In another example aspect, there is provided a gas permeable3D electrode comprising: a gas permeable material, preferably that isnon-conductive and liquid electrolyte impermeable; a porous conductivematerial attached to or positioned adjacent to the gas permeablematerial; and a catalyst in electrical communication with the porousconductive material, where the catalyst may be located on the porousconductive material or on the gas permeable material, or the catalystmay be located on both the porous conductive material and the gaspermeable material. In other example aspects, the porous conductivematerial can be attached to, fixed to, positioned adjacent, orpositioned near with some degree of separation, the gas permeablematerial. In another example aspect, the porous conductive material ispreferably attached to the gas permeable material by using a bindermaterial, which may also be provided with one or more catalysts. The gaspermeable 3D electrode can also be termed a gas permeable composite 3Delectrode.

In a preferred example, the gas permeable material is non-conducting andimpermeable to a liquid electrolyte, during normal operational use ofthe electrode, and the porous conductive material is permeable to theliquid electrolyte. Preferably the gas permeable material is a differentmaterial to the porous conductive material, which are provided as sheetsor layers and laminated together.

Further aspects, details and applications of example 3D electrodes andGDEs that can be utilised can be found in the Applicant's concurrentlyfiled PCT patent applications “Composite Three-Dimensional Electrodesand Methods of Fabrication” filed on 30 Jul. 2014, “ModularElectrochemical Cells” filed on 30 Jul. 2014, and “Electro-Synthetic orElectro-Energy Cell with Gas Diffusion Electrode(s)” filed on 30 Jul.2014, which are all incorporated herein by reference.

The combination of the above properties means that example GDEs of thepresent embodiments can provide an inexpensive, robust, and/ormechanically-strong GDE that has a relatively high wetting pressure andunusually high electrochemical activity. GDEs of this class or type can,consequently, be readily, generally, and beneficially deployed as gaselectrodes in a variety of industrial electrochemical processes anddevices. This additionally provides the ability to manageelectrochemical reactions using the example GDEs.

It has further been realised by the inventors that the unique qualitiesof the developed electrodes or GDEs, along with other physicalproperties, are indicative of a powerful proclivity by electrodes andGDEs of this class or type, to facilitate gas depolarization reactionsat electrodes, for example the counter electrode, in industrialelectrochemical, electro-synthetic and/or electro-energy processes,cells and/or devices. These advantageous properties are believed toarise from the unique features of distinctive 3D electrodes and GDEs.

The porous conductive material can be attached to the gas permeablematerial by being adhered to or laminated to the gas permeable material.Alternatively, the porous conductive material can be provided on the gaspermeable material by being coated on or deposited on at least part ofthe gas permeable material. Alternatively, the gas permeable materialcan be provided on the porous conductive material by being coated on ordeposited on at least part of the porous conductive material.

A key feature of GDEs of this type or class is that they can displaysubstantially high, high or even extraordinarily high wetting pressures.For example, when an ePTFE membrane with about 0.2 μm sized pores areused for the liquid impermeable and gas permeable material of GDEs, thenthe resulting GDEs typically display wetting pressures that are verysimilar to the wetting pressures of the membranes themselves; namely,about 3.4 bar. The addition of a barrier layer or film that permitstransport of the reactant/product gas but excludes water vapour wouldtypically elevate this wetting pressure still further. The result isvery substantially greater wetting pressures than displayed byconventional GDEs, which do not appear to exceed 0.2 bar (see, forexample, recent International Application Publication No. WO 2013037902,which describes the fabrication of a novel GDE with a “record” wettingpressure of 0.2 bar).

In example aspects, industrial or commercial gas-to-liquid and/orliquid-to-gas electrochemical processes can be improved, enhanced ormore optimally managed by:

-   -   1. using GDEs having wetting pressures and/or bubble points        greater than 0.1 bar or greater than 0.2 bar, such as those        described above rather than conventional electrodes, at one or        more, or each, electrode where a liquid-gas interface exists in        the electrochemical cell, and/or    -   2. applying a suitable pressure differential across or between        the liquid-side and the gas-side of the one or more GDEs to        thereby obtain an improved efficiency of the reaction, for        example under conditions where:        -   a. The three-phase solid-liquid-gas interface within the GDE            is maintained in a well-defined, narrow, and/or stable state            during operation. This can be achieved by ensuring that the            pressure applied by the liquid electrolyte on the GDE            relative to the gas side of the GDE, does not exceed the            wetting pressure of the GDE during operation,        -   and/or        -   b. An electrode face of the GDE is maintained as bubble-free            or substantially free of new bubble formation, during            operation. This can be achieved by ensuring that the            pressure applied by the gas on the GDE relative to the            liquid side of the GDE, does not exceed the bubble point of            the GDE during operation.

Thus, it is generally not necessary, nor beneficial to employ complexand expensive pressure-equalising equipment in industrial gas-liquidelectrochemical cells employing GDEs. Instead, an improvement or benefitmay be achieved by using GDEs with relatively high wetting pressuresand/or bubble points, for example in association with relatively simplerpressure monitoring equipment to thereby maintain a suitably effectivepressure differential across the one or more GDEs. During operation,preferably at all times, the pressure differential should be maintainedas less than the wetting pressure of the ODE (when the liquidelectrolyte side has the higher pressure) or its bubble point (when thegas side has the higher pressure).

Accordingly, in various example aspects, there is provided a methodand/or an electrochemical cell to manage gas-to-liquid or liquid-to-gaselectrochemical processes, the method and/or electrochemical cellcomprising or utilising:

-   -   1. The use of one or more gas diffusion electrodes, with a        relatively high wetting pressure and/or a bubble point, for        example greater than about 0.2 bar, where a liquid-gas interface        exists in the electrochemical cell;    -   2. Subjecting the one or more gas diffusion electrodes to a        suitable pressure differential between a liquid side and a gas        side, whilst ensuring that:    -   3. an excess pressure on the liquid side of the GDE over the gas        side, does not exceed the wetting pressure of the GDE during        operation (in cases where the liquid electrolyte side has the        higher pressure); and/or    -   4. an excess pressure on the gas side of the GDE over the liquid        side, does not exceed the bubble point of the GDE (in cases        where the gas side has the higher pressure).

It should be noted that the electrochemical cell according to variousembodiments is not restricted to GDEs of the novel type or classdescribed above. Whilst these GDEs are particularly useful, the methodand electrochemical cell of the present embodiments can be applied toall types of GDEs having wetting pressures and/or bubble points greaterthan about 0.2 bar.

In one embodiment, very substantial improvements may be achieved ingas-to-liquid and/or liquid-to-gas electrochemical processes byequipping the electrochemical cells with suitable GDEs and thenoperating the electrochemical cells with relatively high pressure on theliquid electrolyte side relative to the gas side of the GDE, or on thegas side relative to the liquid electrolyte side. For example, a highpressure differential of greater than or equal to about 2 bar can beemployed. Also for example, a high pressure differential of greater thanor equal to about 1 bar, about 2 bar, about 3 bar, about 4 bar, about 5bar or about 6 bar can be beneficially employed. With suitable GDEs evenabout 7 bar or greater pressure differentials may be achieved and bebeneficial. In all cases however, the pressure differential across theGDE should not exceed: (i) the wetting pressure (when the liquidelectrolyte side has the higher pressure), or (ii) the bubble point(when the gas side has the higher pressure).

In another embodiment, other gas-to-liquid and/or liquid-to-gaselectrochemical processes may be most suitably operated with relativelymoderate pressure on the liquid electrolyte side relative to the gasside of the GDE, or on the gas side relative to the liquid electrolyteside. For example, certain reactions may be most effectively carried outusing GDEs operating at a pressure differential of about 0.2 bar orgreater. Also for example a moderate pressure differential of greaterthan or equal to about 0.2 bar, about 0.3 bar, about 0.4 bar or about0.5 bar can be employed. Once again, preferably in all cases, thepressure differential across the GDE should not at any time exceed: (i)the wetting pressure of the GDE (when the liquid-electrolyte side hasthe higher pressure) or (ii) the bubble point of the GDE (when the gasside has the higher pressure).

In further embodiments, the pressure differential may be set to be aselected pressure differential that is close to, or very close to, butbelow the wetting pressure of the GDE (when the liquid electrolyte sidehas the higher pressure), or the pressure differential may be set to bevery close to, but below the bubble point of the GDE (when the gas sidehas the higher pressure). For example, the pressure differential may beset to about 0.1 bar, about 0.2 bar or about 0.3 bar below the wettingpressure of the GDE (when the liquid electrolyte side has the higherpressure), or the pressure differential may be set to about 0.1 bar,about 0.2 bar or about 0.3 bar below the bubble point of the GDE (whenthe gas side has the higher pressure).

In alternative embodiments designed to provide a margin of error in theevent of sudden and unexpected pressure swings, the pressuredifferential may be set to be a selected pressure differential that issignificantly below the wetting pressure of the GDE (when the liquidelectrolyte side has the higher pressure), or the pressure differentialmay be set to be significantly below the bubble point of the GDE (whenthe gas side has the higher pressure). For example, the pressuredifferential may be set to from about 1 bar to about 2 bar, for exampleabout 1 bar or about 1.5 bar, below the wetting pressure of the GDE(when the liquid electrolyte side has the higher pressure), or thepressure differential may be set to from about 1 bar to about 2 bar, forexample about 1 bar or about 1.5 bar, below the bubble point of the GDE(when the gas side has the higher pressure).

Preferably, but not exclusively, the pressure differential across theGDE is selected so as to improve or maximise the energy efficiency orother benefits of the reaction and minimise the energyconsumption/wastage of the reaction. The selection of the pressuredifferential employed will be typically dependent upon the nature of thereaction itself and on the physical limitations imposed by the GDEsused.

Improved energy efficiencies may derive from the inherent nature of thereaction under different conditions of pressure, or it may derive fromthe circumstances of the reaction. For example, application of themethod and/or electrochemical cell may have the effect of substantiallyeliminating bubbles at the liquid face of the GDE and thereby alsodiminishing the energy-sapping effect created by the bubbleoverpotential and the increased electrolyte resistance arising from thepresence of bubbles. In embodiments where this is achieved, there willbe a decrease in the energy requirement for the process. Other benefits,including but not limited to beneficial industrial utility, may also berealized.

In another example, GDEs of the aforesaid type or class aresignificantly more electrochemically and catalytically active than mayreasonably be expected in that they may spontaneously extract oxygenfrom atmospheric air even though oxygen makes up only 20% of theatmosphere. In so doing, such GDEs may substantially decrease the energyrequirements of the reaction. In an alternative example, GDEs of theaforesaid type or class may facilitate an electrochemical reaction thatwould otherwise not be facilitated or that is not known at the presenttime. In embodiments where such effects are achieved, there willgenerally be a decrease in the energy requirement for the process. Otherbenefits, including but not limited to beneficial industrial utility,may also be realized. For example, it may be enough to use atmosphericoxygen for a reaction rather than needing to provide pure oxygen.Alternatively, a hitherto unknown electrochemical reaction may befacilitated, with beneficial effects.

In still other embodiments, energy savings and other benefits may beachieved by applying a substantial pressure on the liquid electrolyte tothereby produce a pressurised gas product on the gas side of the GDE. Inso doing, one may eliminate or diminish the need for an externalcompressor with which to compress the product gas to a high pressure,thereby decreasing the overall energy requirements of the process. Otherbenefits, including but not limited to beneficial industrial utility,may also be realized.

It should be noted that, while raising the pressure on the liquidelectrolyte side provides potentially significant benefits, it may alsohave the effect of making it difficult to manage the pressure dropacross the GDE. For example, if the product side is unexpectedly drawndown (e.g. a large quantity of gas is taken out), then the pressure onthe product side may suddenly drop and the differential pressure willsuddenly increase. An example embodiment provides a method and/orelectrochemical cell for dealing with such occurrences, especially whenGDEs with high wetting pressures and bubble points are used, since theyprovide more flexibility to operate the cell with the electrolyte at anelevated pressure. In industrial electrochemical processes of this type,it is important to have controls in place to enable the use ofdifferential pressures and to use the controls to manage thedifferential pressure, along with a good GDE that can handle arelatively high differential pressure.

In a further embodiment, the electrochemical cell/method is applied orused in a reversible fuel cell, to control the differential pressureduring electrolysis and, optionally and additionally, to manage thedifferential pressure during the operation of the fuel cell, so as toobtain high gas transfer into the electrolyte without substantial bubbleformation. Further details of this application are described in theApplicant's concurrently filed PCT application “Modular ElectrochemicalCells” filed on 30 Jul. 2014, which is incorporated herein by reference.

In further embodiments, high gas pressures relative to the liquidelectrolyte pressures may serve to increase the solubility of a reactantgas in the liquid electrolyte and thereby increase the energy or otherefficiency of the reaction. Other benefits, including but not limited tobeneficial industrial utility, may also be realized.

Energy efficiencies and other benefits of the type described above, maybe further improved by: (i) the use of relatively low current densities,which minimise electrical losses and thereby maximise electricalefficiency, (ii) the use of highly-efficient catalysts, including, butnot limited to low-cost catalysts comprising of Earth-abundant elementswhich operate highly efficiently at lower current densities, (iii)optimizing the inter-electrode distance, (iv) using appropriatelydesigned current distributors and collectors, and/or (v) improving masstransfer.

Further ways in which the 3D electrodes or GDEs can be used to improveenergy efficiency or industrial utility are described in the Applicant'sconcurrently filed PCT application “Electro-Synthetic or Electro-EnergyCell with Gas Diffusion Electrode(s)” filed on 30 Jul. 2014, which isincorporated herein by reference.

Embodiments also address the pressing need for electrochemical cellscapable of performing gas-to-liquid or liquid-to-gas transformationswith high energy efficiencies and low cost, preferably in a modular orrelatively small-scale, “on-site” form.

The low cost may be achieved by the combination of: (i) low-costbreathable materials as the substrate for GDE anodes and/or cathodes ofthe electrochemical cell, (ii) low-cost metallic elements for theconductive portion of the GDE, (iii) compact and inherently efficientcell designs, (iv) inherently inexpensive assembly techniques, (v) theuse of low-cost catalysts comprising of Earth-abundant elements, as thecatalysts at the anode and cathode, (vi) low-cost reactor arrangementsthat have large electrode surface areas but small external footprints,(vii) eliminating the need for a diaphragm between the electrodes (asare required in many electrochemical cells), and/or (viii) optimalelectrode manufacturing methods. Other means of achieving low-cost andrealising other benefits may also be envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments will now be described solely by way ofnon-limiting examples and with reference to the accompanying figures.Various example embodiments will be apparent from the followingdescription, given by way of example only, of at least one preferred butnon-limiting embodiment, described in connection with the accompanyingfigures.

FIG. 1 (prior art) depicts in schematic form, a conventional gasdiffusion electrode. The lower part of the figure is a magnified view ofa section of the conventional gas diffusion electrode.

FIG. 2 depicts in schematic form, an example 3D electrode, or gasdiffusion electrode, according to the present embodiments (not toscale). The lower part of the figure is a magnified view of a section ofthe gas diffusion electrode.

FIG. 3 depicts a schematic cross-sectional view of an example GDE (notto scale).

FIG. 4 depicts a schematic side view of an example GDE in which the twoouter surfaces are both conductive (not to scale).

FIG. 5 depicts a disassembled example electrochemical cell used to studythe effect of pressure differentials on electrochemical cells containingGDEs with 3.4 bar wetting pressures and 2 bar bubble pressures. Anelectrode is placed between the left-most component and the centercomponent, and another electrode is placed between the right-mostcomponent and the center component, when assembled.

FIG. 6 depicts the cell from FIG. 5 when assembled and configured tocontain two GDEs (having wetting pressures of 3.4 bar and bubblepressures of 2 bar each), and showing gas regulating attachments to acentral liquid electrolyte chamber on one side of the GDEs, and to theouter gas chambers on the other sides of the GDEs.

FIG. 7 depicts the effect on the electrochemically measured rate ofreaction (left axis; in mA/cm²) for a water electrolysis reaction in thecell of FIG. 6, as a function of the excess (differential) pressure onthe liquid electrolyte relative to the gas side of the GDEs (bottomaxis; in kPa).

FIG. 8 depicts the effect on the electrochemically measured rate ofreaction (left axis; in mA/cm²) for a water electrolysis reaction in thecell of FIG. 6, as a function of the applied pressure on the liquidelectrolyte, whilst maintaining a constant excess (differential)pressure of 0.5 bar on the liquid electrolyte relative to the gas sideof the GDEs.

FIG. 9 depicts a 3D graph relating the effect on the electrochemicallymeasured rate of reaction (left axis; in mA) for a water electrolysisreaction in the cell of FIG. 6, as a function of the applied pressure onthe liquid electrolyte, whilst maintaining a constant excess(differential) pressure of 0.5 bar on the liquid electrolyte relative tothe gas side of the GDEs (bottom axis; in kPa) versus the appliedvoltage (orthogonal axis; in V).

FIG. 10 schematically illustrates an example of how one or more flexible3D electrodes, used as a GDE, can be rolled or spiral-wound.

FIG. 11 schematically illustrates an example of how flexible 3Delectrodes, used as GDEs, for example after being stacked or layered asanode-cathode pairs, can be formed into an example spiral-wound cell ordevice.

EXAMPLES

The following modes, features or aspects, given by way of example only,are described in order to provide a more precise understanding of thesubject matter of a preferred embodiment or embodiments.

A New Approach to Making 3D Electrodes and Gas Diffusion Electrodes(GDEs)

FIG. 2 illustrates in schematic form the general structure of an example3D electrode or GDE 115 that can be used in present embodiments. A 3Delectrode or GDE 115 of the present embodiments differs from aconventional 3D particulate fixed bed electrode or GDE 110 in that itseparates the features of hydrophobic pore structure and conductivity,preferably catalytic conductivity, into two distinct regions, each ofwhose properties improve upon and may be more fully controlled than ispossible in a conventional 3D particulate fixed bed electrode or GDE. Insome embodiments more than two distinct regions may be possible. Thus,an example embodiment of a 3D electrode or GDE 115 may comprise of aliquid-and-gas-porous conductor 130 (i.e. a porous conductive material),that is preferably also provided with a catalyst, coupled with, attachedto, abutting, or positioned adjacent a non-conductive gas permeablematerial 120, that is also preferably liquid electrolyte impermeable,e.g. strongly hydrophobic. The gas permeable material 120 and conductor130 (i.e. porous conductive material) are substantially distinct,demarcated or separated, thereby providing a first region 135(conductive region) and a distinct second region 125 (gas permeableregion), respectively. The gas permeable material 120 and the conductor130 are preferably positioned adjacent, abut, touch or adjoin eachother, so that there can be touching or overlap of a periphery of theregions at a boundary region or interface 140. The non-conductive,hydrophobic, gas permeable material 120 may display pores that arebetter defined, more uniform, and potentially of smaller average size,than can be achieved in a conventional 3D electrode or GDE. Theliquid-and-gas-porous conductor 130 may, similarly, be more conductivethan a conventional 3D electrode or GDE. The low hydrophobicity of theliquid-and-gas-porous conductor (i.e. porous conductive material) 130will usually also see it completely or substantially completely filledwith liquid electrolyte under normal operating conditions, therebymaximally facilitating catalysis. By contrast, the liquid impermeabilityor high hydrophobicity of the non-conductive, gas permeable material 120will typically see it completely empty or substantially empty of liquidelectrolyte at atmospheric pressure, thereby maximally facilitating gastransport into and out of the GDE 115.

The gas permeable 3D electrode 115 thus provides a gas permeablematerial 120 that is non-conductive, and a porous conductive material130 attached to the gas permeable material 120. In operation, the gaspermeable material 120 faces a gas side of a cell and the porousconductive material 130 faces a liquid electrolyte side of the cell. Inuse, a three-phase solid-liquid-gas boundary is able to form at or neara surface 122 of the gas permeable material 120 facing the porousconductive material 130.

The porous conductive material 130 is coupled to, touching, positionedadjacent, attached to or abutting the non-conductive gas permeablematerial 120, which may be hydrophobic, to form or provide an interface140 (or boundary region) of or between the porous conductive material130 and the non-conductive gas permeable material 120. Preferably, thisprovides two regions (a first region 135 including the porous conductivematerial 130 and a second region 125 including the non-conductive gaspermeable material 120) that are distinct, demarcated or separated.Preferably, the first region 135 and the second region 125 arepositioned adjacent, abut, touch or adjoin each other, so that there isan interface 140 (or a boundary region) for the first region 135 and thesecond region 125. Thus, in operation of a preferred embodiment, athree-phase solid-liquid-gas boundary forms at or near the surface 122of the gas permeable material 120 facing the porous conductive material130, which may also be at or near the interface 140 (i.e. at or within aboundary region) between the first region 135 (i.e. the porousconductive material 130, which can include a catalyst) and the secondregion 125 (i.e. the non-conductive gas permeable material 120). In oneexample, the solid-liquid-gas boundary, which is formed during use ofthe electrode in a cell or reactor, has a macroscopic width that issubstantially two-dimensional in relation to the width or thickness ofthe electrode 115. In another example, the solid-liquid-gas boundary isformed at the interface 140 of the gas permeable material 120 and theporous conductive material 130.

When such a 3D electrode or GDE 115 is contacted on the conductive sideby a liquid electrolyte and on the non-conductive side by a gaseousmaterial, then the above physical features cause the formation of athree-phase solid-liquid-gas boundary at or near the surface 122 (orinterface 140 between the two regions). The three-phase solid-liquid-gasboundary is quite different to that formed in a conventional 3Delectrode or GDE. The boundary differs in that it is far better defined,narrower, more stable and/or more robust than can be achieved in aconventional 3D electrode or GDE. For example, the three-phasesolid-liquid-gas boundary formed at or near surface 122, oralternatively at or near interface 140, has a macroscopic width that istwo-dimensional or substantially two-dimensional in relation to thewidth of the electrode 115.

These features are important because the inventors have found thatexample embodiment 3D electrodes or GDEs, such as GDE 115, may, whenfabricated in a carefully calibrated way, combine at the interface 140between gas permeable material 120 and conductor 130, an enhanced oroptimum hydrophobic pore structure that facilitates enhanced or maximumgas transport, with an enhanced or optimally conductive, increased ormaximally catalytic structure. In effect, at the three-phasesolid-liquid-gas boundary in example embodiment 3D electrodes or GDEs,such as GDE 115, each of the critical properties of the electrode may bemade ideal, or, at least, nearer to ideal than is otherwise possible.

The effect of this optimisation can be remarkable and unexpectedlysignificant. Despite being narrower and confined to what appears to be,macroscopically, a 2D geometry, the electrochemical capabilities of thethree-phase solid-liquid-gas boundary in example embodiment 3Delectrodes or GDEs, such as GDE 115, may substantially improve upon and,in fact, far exceed those of conventional 3D electrode or GDEs, such asGDE 110.

This is because the fabrication of conventional 3D electrodes or GDEs,as currently employed in the art, is predicated on creating all of theimportant physical properties at the same time within a single material.This approach effectively ignores the fact that the key properties of 3Delectrodes or GDEs (namely: pore structure, hydrophobicity, gastransport, liquid transport, conductivity and catalytic activity) aretypically inter-dependent and therefore not open to ready, concurrentoptimisation within a single material. Example embodiment 3D electrodesor GDEs 115 take account of this limitation and separately optimise thekey properties, to thereby achieve more optimum overall properties atthe interface 140 between the gas permeable layer 120 and the conductivelayer 130.

The inventors have further found that the three-phase solid-liquid-gasboundary may, in fact, at a microscopic level comprise a contorted 3Dstructure with an unexpectedly large overall surface area. This isparticularly the case if the conductive region 135 overlaps somewhatwith the gas permeable region 125.

These very fundamental enhancements may impart example embodiment 3Delectrodes or GDEs, such as GDE 115, with a range of unexpected andnovel electrochemical and physical capabilities. These include:

-   -   1. much higher wetting pressures and bubble points than can be        achieved in conventional 3D electrodes or GDEs. “Wetting        pressure” is defined as the lowest excess of pressure on the        liquid electrolyte side of a 3D electrode or GDE relative to the        gas side of the electrode, at which the liquid electrolyte        penetrates and floods the electrode. The “bubble point” is        defined as the lowest excess of pressure on the gas side of a 3D        electrode or GDE relative to the liquid electrolyte side of the        3D electrode or GDE, at which the gas blows through the        electrode and forms bubbles at the electrode surface on the        liquid electrolyte side. Example embodiment 3D electrodes or        GDEs, such as GDE 115, typically have wetting pressures and        bubble points in excess of 0.2 bar, whereas conventional 3D        electrodes or GDEs, such as GDE 110, typically have wetting        pressures and bubbles points of 0.2 bar or less;    -   2. lower electrical resistances, higher electrocatalytic        activities and reactivities, as well as more efficient        utilization of catalytic materials, than can be realised in        conventional 3D electrodes or GDEs, especially, but not        exclusively, when operated at relatively low current densities;        and    -   3. an apparent capacity to facilitate hitherto unachievable        gas-to-liquid or liquid-to-gas electrochemical reactions, or, at        least, improve upon electrochemical reactions that have not        proved practically viable to date, especially, but not        exclusively, when operated at relatively low current densities.        Examples of such transformations include the electrochemical        production of hydrogen peroxide from caustic and air oxygen, the        production of pure oxygen from air oxygen, the operation of fuel        cells with high energy efficiencies, and the direct generation        of electrical current by the reaction of methane within a direct        methane fuel cell.

Additionally, example embodiment 3D electrodes or GDEs, such as GDE 115,are flexible and may be double-sided, allowing them to be deployed indensely-structured, flexible, spiral-wound and other electrochemicalcells, for example of the types described in the Applicant'sconcurrently filed PCT patent application “Modular ElectrochemicalCells” filed on 30 Jul. 2014, which is incorporated herein by reference.

Example embodiment 3D electrodes or GDEs, such as ODE 115, may also befabricated in an exceedingly low cost manner, allowing for the practicaluse of: (i) relatively low current densities, which minimise electricallosses and thereby maximise electrical efficiency, and (ii) low-costcatalysts comprising of Earth-abundant elements which only operateefficiently at lower current densities. By these means, it becomespossible to manufacture practically and economically viable, large-scaleelectrochemical cells for use in industrial-scale electro-synthetic andelectro-energy applications. Such cells may achieve energy efficienciesthat have hitherto been unavailable in large-scale production and energyenvironments. For example, chlorine may be manufactured at scale usingthe chlor-alkali process with 91% energy efficiency, whereas the bestavailable industrial chlor-alkali plants achieve 66% energy efficiency.

The higher wetting pressures that can be achieved in example embodiment3D electrodes or GDEs, such as GDE 115, relative to conventional GDEs,such as GDE 110, allow for the direct production of pressurised gases inlarge-scale, industrial liquid-to-gas electro-synthetic/electro-energycells without the risk of the electrodes becoming flooded andelectrolyte leaking out of the electrolyte chamber (‘flooding-free’operation). The higher bubble points that can be achieved means thatreactant gases may be introduced at pressure into large-scale,industrial gas-to-liquid electro-synthetic/electro-energy cells via gasdiffusion electrodes, without forming energy-sapping bubbles in theliquid electrolyte (‘bubble-free’ operation).

The present embodiments teach the approach of harnessing an interfacebetween a liquid-and-gas-porous conductive layer and a gas permeable,hydrophobic layer to achieve practical and economic advantages such asthose described above. Such advantages are achieved when the regions 125and 135 are carefully designed/selected, fabricated in a calibrated wayand located in close proximity to each other. That is, the three-phasesolid-liquid-gas boundary should be enhanced or optimised, typicallythrough carefully calibrated fabrication in order to improve uponconventional GDEs.

Fabrication of 3D Electrodes and GDEs

As noted above, a new approach to developing 3D electrodes or GDEsinvolves separately enhancing or optimising one or more key features of3D particulate fixed-bed electrodes and gas diffusion electrodes indifferent locations and then combining the enhanced or optimisedcomponents along an interface. Thus, for example, the properties ofhydrophobicity and porosity to the liquid electrolyte may be optimisedin a non-conductive layer. This layer may then be combined along orabout an interface, with a separate porous conductive layer in which theconductance and catalytic properties have been optimised.

The hydrophobic material may be a commercially available expanded PTFEmembrane having high hydrophobicity and a substantially uniform poresize. Such membranes are manufactured to more accurate specificationsthan are possible in conventional 3D particulate fixed bed electrodes orGDEs.

The conductive material may be a metallic material, such as a metal meshor grid (decorated or coated with a catalyst-binder mixture), that isinherently more conductive than the carbon black used in conventional 3Dparticulate fixed bed electrodes or GDEs. The porous conductive metalmay be selected based on hydrophobicity to match a liquid electrolyte.

Small amounts of PTFE and carbon black may be used in the fabrication ofthe 3D electrode or GDE; for example in a binder material to bind thecatalyst in the conductive layer to the metallic material. A keydifference from conventional 3D particulate fixed-bed electrodes andGDEs is, however, that the PTFE and carbon black do not form asuperstructure within which a three-way solid-liquid-gas boundary may beformed. Instead, the solid-liquid-gas boundary is formed at or near asurface of the gas permeable material facing the porous conductivematerial, or in another example this could be said to be at or near theinterface between the hydrophobic porous region and the conductiveregion.

The inventors have studied such interfaces in 3D electrodes and GDEs anddiscovered that they yield surprisingly and unexpectedly effectiveelectrochemical systems. Their efficacy appears to derive from theirunique architecture, which is brought about by careful and calibratedconstruction. For improved performance, this might be coupled withoperation of the 3D electrodes at low current density (at moderatevoltages), such as from 1 mA/cm² to 500 mA/cm² or, preferably, from 1mA/cm² to 200 mA/cm², or preferably from 1 mA/cm² to 100 mA/cm²,inclusively.

General Example Embodiments

A new approach to developing 3D electrodes that can be used as GDEsinvolves adapting existing, commonly available porous materials so thatthey may act as practically useful 3D electrodes and GDEs.

In a preferred example there is provided a 3D electrode which includes agas permeable material that is liquid impermeable, during normaloperational use of the electrode, a porous conductive material at leastpartially covering the gas permeable material (such as covering one sideor part of one side of the gas permeable material) that is liquidpermeable and gas permeable, and a binder material which adheres orattaches the gas permeable material and the porous conductive materialto each other. The binder material (which may be a mixture of materials)penetrates the gas permeable material to a depth less than the thicknessof the gas permeable material. In one example, the binder material canbe present between the porous conductive material and the gas permeablematerial. In another example, the binder material is present at aninterface or boundary region of the porous conductive material and thegas permeable material. In another example, the binder material adjoinsthe porous conductive material with the gas permeable material.

Thus, a porous conductive material (e.g. a conductive metallic layer) isprovided at or near one surface of the 3D electrode and a gas permeablematerial (e.g. a non-conductive layer) is provided at or near the other,opposing, surface of the 3D electrode. The conductivity of the resultingcomposite 3D electrode thus varies along the thickness of the 3Delectrode. The porous conductive material (e.g. conductive metalliclayer) is gas permeable and at least partially, preferably fully, liquidpermeable, whereas the gas permeable material (e.g. non-conductivelayer) is gas permeable and liquid impermeable, during normaloperational use of the electrode. The porous conductive material (e.g.conductive metallic layer) can be in one example part of an outersurface of the 3D electrode and is relatively less hydrophobic than thegas permeable material, whereas the bulk 3D electrode is gas breathableand liquid impermeable.

When the 3D electrode is in use, for example as a GDE, a three-phasesolid-liquid-gas boundary is formed within the 3D electrode, preferablyat or near the surface of the gas permeable material that faces theporous conductive material. The solid-liquid-gas boundary is narrow inmacroscopic width compared to the thickness of the electrode or of thegas permeable material. Preferably, the maximum width of thesolid-liquid-gas boundary is two-dimensional or substantiallytwo-dimensional in relation to the width (or thickness) of the 3Delectrode, or in relation to the width (or thickness) of the gaspermeable material. In another example, the maximum width of thesolid-liquid-gas boundary is less than or equal to the thickness of theapplied binder material in the boundary region or interface between thegas permeable material and the porous conductive material.

The solid-liquid-gas boundary is narrow compared to the width of theelectrode. This can depend on the width of the electrode materials usedand the application. In one example the solid-liquid-gas boundary canhave a maximum (or macroscopic) width of less than 400 μm. In otherexamples, the solid-liquid-gas boundary can have a maximum (ormacroscopic) width of less than about 300 μm; or less than about 200 μm;or less than about 100 μm; or less than about 50 μm; or less than about10 μm; or less than about 1 μm; or less than about 0.1 μm; or less thanabout 10 nm. By contrast, conventional gas diffusion electrodestypically have their solid-liquid-gas boundaries distributed overthicknesses of from 0.4 mm to 0.8 mm in the case of fuel cell gasdiffusion electrodes, or even greater, such as several millimeters inindustrial electrochemical gas diffusional electrodes.

In other examples, the maximum width of the solid-liquid-gas boundarycan be defined in relation to the width of the electrode, or in relationto the width of one of the constituting materials or layers. In oneexample the solid-liquid-gas boundary can have a maximum width of lessthan about 30% of the width of the electrode. In other examples, thesolid-liquid-gas boundary can have a maximum width of less than about20% of the width of the electrode; or less than about 15% of the widthof the electrode; or less than about 10% of the width of the electrode;or less than about 5% of the width of the electrode; or less than about1% of the width of the electrode; or less than about 0.1% of the widthof the electrode; or less than about 0.01% of the width of theelectrode.

Preferably, though not necessarily, the porous conductive material is apure or highly purified metal. For example, the porous conductivematerial can be, but is not limited to pure or purified nickel orStainless Steel. Alternatively, the porous conductive material can be ametal such as Ti, Cr, Pt, Cu, Pb, Sn, Co, Mn, Au or Ag, or mixtures oralloys thereof. Alternatively, the porous conductive material could be ametal coated with another metal. For example, the porous conductivematerial could be stainless steel coated with nickel. Alternatively, theporous conductive material could be stainless steel coated with Ti, Cr,Pt, Cu, Pb, Sn, Co, Mn, Au or Ag. In further examples, the porousconductive material may be a polymer coated with a conductive layer or ametallic layer, such as a polymer fabric coated with a metallic layer.In still other examples, the porous conductive material may be formallynon-metallic in character but display properties of electricalconduction which are similar to those of metals; for example, carbonfibre or carbon cloth materials.

In some examples, the conductive region or portion (which can includethe porous conductive material and a binder material if used) of the 3Delectrode comprises less than or equal to about 10% carbon atoms, orless than or equal to about 20% carbon atoms, or less than or equal toabout 30% carbon atoms. The carbon atoms can be provided as part of, orattached to, the porous conductive material, and/or included as acomponent of the binder material, in which case the conductive region orportion is provided by the porous conductive material and the bindermaterial. This can provide a significant benefit, as carbon is lessexpensive than metals and also lighter. In another example, theconductive region or portion of the 3D electrode can comprise activatedcarbon. In these examples, the conductive region or portion is notsimply a continuous metal or continuous metal coating, such as would beobtained from metallic sputter coating. A benefit of using activatedcarbon is that some catalysts, such as nano-catalysts, can betterassociate with or bind to the activated carbon than compared to metals.

In one example, the porous conductive material is stainless steel mesh,for example 100 lines per inch (LPI) stainless steel mesh (thicknessabout 60-80 micron), which is applied by lamination at, for example, atemperature of 50° C. and a pressure of 500 kPa to a polymer membrane ofexpanded PTFE (ePTFE) that has been pre-coated by screen-printing, witha layer about 20 micron thick of a binder mixture that comprises carbonblack (about 10% by weight), nickel nanoparticles (about 80% by weight),and an ionomer, such as a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer (e.g. Nafion™ material), (about 10% by weight).

In other examples, the layer of binder material can be from about 1micron to about 100 microns thick, or about 10, about 30, about 40,about 50, about 60, about 70, about 80, about 90, or about 100 micronsthick. The binder material may comprise:

carbon black (from about 1% to about 30% by weight, or from about 1% toabout 20% by weight, or from about 1% to about 10% by weight, or fromabout 1% to about 5% by weight, or about 5%, or about 10%, or about 15%,or about 20%, or about 25%, or about 30% by weight),

nickel particles or nanoparticles (from about 1% to about 90% by weight,or from about 1% to about 80% by weight, or from about 1% to about 70%by weight, or from about 1% to about 60% by weight, or from about 1% toabout 50% by weight, or about 10%, or about 20%, or about 30%, or about40%, or about 50%, or about 60%, or about 70%, or about 80%, or about90% by weight), and/or

an ionomer, such as a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer (e.g. Nafion™ material), (from about 1% to about30% by weight, or from about 1% to about 25% by weight, or from about 1%to about 20% by weight, or from about 1% to about 10% by weight, or fromabout 1% to about 5% by weight, or about 5%, or about 10%, or about 15%,or about 20%, or about 25%, or about 30% by weight).

In another example, the gas permeable material is a porous polymermembrane or structure. In another example the gas permeable material canbe made or formed of one or more substances selected from, but notlimited to the group of PTFE, polypropylene, polyethylene orpolysulfone. The gas permeable material can be any medium, article,layer, membrane, barrier, matrix, element or structure that issufficiently porous or penetrable to allow movement, transfer,penetration or transport of one or more gases through or across at leastpart of the material, medium, article, layer, membrane, barrier, matrix,element or structure (i.e. the gas permeable material). That is, asubstance of which the gas permeable material is made may or may not begas permeable itself, but the material, medium, article, layer,membrane, barrier, matrix, element or structure formed or made of, or atleast partially formed or made of, the substance is gas permeable. Thegas permeable material can also be referred to as a “breathable”material. By way of example only, a gas permeable material can be aporous membrane and a substance from which the gas permeable material ismade or formed can be a polymer, such as PTFE. In one example the 3Delectrode is a Gas Diffusion Electrode.

Preferably, the gas permeable material has substantially uniform poresize. Between the porous conductive material (e.g. conductive metalliclayer) and the gas permeable material (e.g. non-conductive polymerlayer) is a binder material providing a binder layer in a boundaryregion, and on both sides of the boundary region the pores aresubstantially uniform in size and distribution. For example, the averagepore size can be between about 10 nm to about 500 nm, or preferablybetween about 50 nm to about 500 nm, or preferably between about 100 nmto about 500 nm, or in more specific examples about 0.1, 0.2, 0.3, 0.4or 0.5 microns in size. In a most preferred example, the gas permeablematerial has an average pore size of about 50 nm to about 500 nm and isformed of PTFE.

For example, a commonly available and relatively inexpensivenon-conductive porous material is made or formed of “expanded PTFE”,also known as ePTFE (where PTFE=polytetrafluoroethylene). ePTFEcomprises a highly porous (typically 60-80% porosity) fibrous network ofmicroscopically small, hydrophobic PTFE. An important property of ePTFEis that it is highly porous but also highly hydrophobic. Otherwidely-available, commodity-type porous polymer membranes, are made orformed from, but are not limited to, polypropylene, polyethylene,polysulfone, and other polymers of similar ilk.

It should be noted that, while the brand name Goretex® polymer materialcan be used, the inventors have found that use of Goretex® polymermaterial is not preferred or optimum in the applications describedbelow. In fact, the inventors have found that ePTFE membranesmanufactured by the General Electric Company, having some differentproperties, offer the best and most optimum utility in mostelectrochemical devices.

In one example, the depth to which the binder material penetrates thegas permeable material (e.g. polymer layer) is in the range of about 1nm to about 10 μm, or about 50 nm to about 1 μm, or about 50 nm to about500 nm. In a specific example, the porous conductive material is anickel mesh of 100 LPI (LPI=lines per inch), the gas permeable materialis a 0.2 micron PTFE membrane and the binder material is a combinationof carbon black (about 10% by weight), nickel nanoparticles (about 80%by weight), and a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer (e.g. Nafion™ material) (about 10% by weight),and the binder material penetrates the gas permeable material to a depthgreater than 0 but less than the thickness of the gas permeablematerial, for example less than about 850 nm.

In another form there is provided a method of fabricating a 3Delectrode. The steps include selecting a gas permeable material, forexample with a substantially uniform pore size, and then applying, undersuitable (‘calibrated’) heat and pressure for lamination, a porousconductive material to partially coat the gas permeable material, withuse of a binder material as an adhesive. The binder material preferablypenetrates the gas permeable material to a depth less than the thicknessof the gas permeable material.

The (‘calibrated’) lamination step can include: a particular heat orrange of heats of application; a particular pressure or range ofpressures of application; a particular time or period of application;and/or a particular circumstance or range of circumstances ofapplication.

Attachment of one or more porous conductive materials, for example asone or more porous conductive layers or meshes, to the gas permeablematerial, for example a porous polymer membrane, using controllablelamination techniques are employed to produce 3D electrodes. When formedin this way, 3D electrodes with unexpected and improved electrochemicalperformance may be realised, especially relative to other 3D electrodesand to the cost of manufacture. Further, unrelated materials, forexample including catalytic or other materials, can be convenientlyadded to, or formed upon or in-between the one or more porous conductivematerials, layers or meshes, and the gas permeable material to produce3D electrodes that are practical and useful in electro-energy orelectro-synthetic applications. The availability of such 3D electrodesmakes viable or improves the practicality of a range of electro-energyand electro-synthetic applications. Such applications are typicallyunviable or relatively less practical using conventional particulatefixed-bed or gas diffusion electrode technologies.

The porous conductive materials, for example provided as meshes,membranes or layers, can be applied to one or more gas permeablematerials, for example provided as meshes, membranes or layers, having aspecific, and preferably narrow, range of pore sizes, such as the widelyavailable and relatively low cost polymer membranes used in the waterpurification industry. Such membranes are manufactured to contain veryspecific and narrow ranges of pore sizes. By adapting or modifying suchmembranes or other gas permeable materials to be 3D electrodes, one mayconveniently impart upon the 3D electrode highly desirable and otherwiseunobtainable pore properties. For example, 3D electrodes may beconveniently and reliably fabricated with tiny (for example less than500 nm in size) and reasonably or substantially uniform pores that arenot easily, reliably, or inexpensively achieved in conventional 3Delectrodes. Additionally, the desired pore size can be readily varied byselecting a different gas permeable material, for example provided as amembrane or mesh, for adaption or modification into a 3D electrode. Gaspermeable materials with a wide variety of pore sizes are readilyavailable.

A porous conductive material, for example a conductive metallicmaterial, mesh or layer, can be applied such that the produced 3Delectrodes, which can be used as GDEs, display unusually highelectrochemical activities as a function of the electrochemical surfacearea present.

General Example Embodiments—Gas Diffusion Electrode (GDE)

When intended to be used in a Gas Diffusion Electrode (GDE) typeapplication, the porous conductive material (e.g. metallic material orlayer) is preferably, but not exclusively, applied such that theproduced 3D electrodes create uniquely well-defined, narrow and stablethree-way solid-liquid-gas boundaries. In a particular example, theporous conductive material may have a thickness in the range of about 1nm to about 1000 μm, or in the range of about 1 μm to about 100 μm, orin the range of about 5 μm to about 40 μm. By controlling the pore sizeof the gas permeable material (e.g. a polymer layer), one may alsocontrol important physical properties of the 3D electrode, for example a3D GDE, such as the wetting pressure, the bubble point, and thepermeability to gases.

In an example embodiment in the case where a GDE is manufactured using apreviously formed polymer membrane as the gas permeable material, theGDE can have substantially the same wetting pressure as that of thepolymer membrane (i.e. the gas permeable material) used. In the examplecase where a membrane having average pore size 0.2 μm is used as the gaspermeable material in the GDE, the wetting pressure of both the membraneand the GDE is 3.4 bar (such an example polymer membrane is availablefrom the General Electric Company). Thus, liquid water will onlypenetrate and flood the GDE upon the application of 3.4 bar of pressureon the liquid side. The addition of a dense, thin film that is,nevertheless porous to gases but not to liquid water, on top of the PTFEmay increase the wetting pressure to 10 bar or greater. By contrast, tothe knowledge of the inventors all other known GDEs have wettingpressures that currently do not exceed 0.2 bar. Thus, in one form thepresent example electrode has a wetting pressure above 0.2 bar, andpreferably about 3.4 bar or greater.

In preferred examples the porous conductive material is attached to thegas permeable material (e.g. the polymer layer) by being physically(e.g. mechanically) or chemically bonded to the gas permeable material.This can be achieved by the presence of a binder material, or materials,that act to bind the porous conductive material and the gas permeablematerial together. The binder material may be present everywhere,substantially everywhere or almost everywhere between the porousconductive material and the gas permeable material. Alternatively, thebinder material may be present at a selection of spots or regionsbetween the porous conductive material and the gas permeable material.The binder material or materials may further be applied in a pattern tothereby securely attach the porous conductive material to the gaspermeable material. The binder material may include, substantially orpartially, the material which forms the gas permeable material, forexample the polymer material which forms the polymer layer.Alternatively, binder material may be a mixture and comprise one or moreunrelated materials which may concurrently impart one or more otherdesirable properties upon the binder mixture, such as also being aconductor of electricity or a catalyst.

In one example, the binder material attaches to the surface of theporous structure of the gas permeable material (e.g. polymer material orlayer). In another example, the binder material penetrates the porousstructure of the gas permeable material (e.g. polymer material or layer)to a depth less than the thickness of the gas permeable material (e.g.polymer material or layer).

Example gas permeable or breathable 3D electrodes can be formed bydepositing a catalyst within a binder material (e.g. binder layer) on agas permeable material, followed by attaching or laminating thereto, aporous conductive material. In one example, one could start with a gaspermeable non-conductive material and then form thereupon, a bindinglayer using a binder material containing one or more catalysts. To thiscombination, a porous conductive material may be laminated usingsuitable heat and/or pressure.

In a preferred example the 3D electrode is flexible. The porousconductive material or layer can be made at least partially or whollyfrom a substance and/or in a form that is flexible. The gas permeablematerial can similarly be made at least partially or wholly from asubstance and/or in a form that is flexible. Optionally, the gaspermeable material is made at least partially or wholly from a polymeror a combination of polymers, for example PTFE, “expanded PTFE” (ePTFE),polyethylene or polypropylene. The polymer itself may or may not be gaspermeable. For example, the polymer itself may not be gas permeable buta structure or membrane formed from the polymer is gas permeable.

Numerous other industrial electrochemical processes may benefit from theuse of gas depolarized GDEs, if they were practically viable. Theseinclude the electrochemical manufacture of: (a) hydrogen peroxide, (b)fuels from CO₂, (c) ozone, (d) caustic (without chlorine), (e) potassiumpermanganate, (f) chlorate, (g) perchlorate, (h) fluorine, (i) bromine,(j) persulfate, and others. Electrometallurgical applications, such asmetal electrowinning, could also benefit from the energy savingsassociated with anode depolarization; metal electro-deposition occurs atthe cathode side of such cells, while oxygen is evolved at the anode. Ifoxygen evolution was replaced by hydrogen oxidation on a suitable gasdiffusion anode, this would generate substantial energy savings.However, the mechanical characteristics of conventional GDEs make themunsuitable for delimiting narrow-gap chambers, thereby restricting theirapplication in the undivided electrolysis cells that are widely used inelectrometallurgical processes. Moreover, conventional GDEs would leakunder the hydraulic head of electrolytic solutions commonly used inindustrial size electrolysers. Several industrial electrochemicalprocesses in the pulp and paper industry may also benefit from the useof alternative GDEs that could be gas depolarized and withstand a higherpressure differential, including: (a) “black liquor” electrolysis, (b)“Tall Oil recovery” and (c) chloride removal electrolysis. Flooding ofGDEs after the build-up of even very mild liquid pressures is,furthermore, a particular and well-recognized problem in fuel cells,such as hydrogen-oxygen fuel cells.

Thus, the electrochemical cell can be used in the electrochemicalmanufacture of: (a) hydrogen peroxide, (b) fuels from CO₂, (c) ozone,(d) caustic (without chlorine), (e) potassium permanganate, (f)chlorate, (g) perchlorate, (h) fluorine, (i) bromine, (j) persulfate,(k) chlorine, (l) caustic (in general), (m) CO₂ from methane, andothers.

In alternative examples, the electrochemical cell involveselectrochemical processes unique to particular industries. Examplesinclude:

-   -   (i) electrometallurgical applications, such as metal        electrowinning;    -   (ii) pulp and paper industry applications, such as: (a) “black        liquor” electrolysis, (b) “Tall Oil recovery” and (c) chloride        removal electrolysis; and    -   (iii) fuel cell and related device applications, such as        hydrogen-oxygen fuel cells, including but not limited to        alkaline fuel cells.

In another example aspect, the beneficial effect/s may be achieved bythe fact that GDEs according to example embodiments make it possible andpractical to carry out entirely new chemical processes, either in cellsor devices. For example, hitherto unconsidered processes for theformation of fuels from carbon dioxide, or remediation of SO_(x) andNO_(x) pollution, are possible and practical using GDEs according toexample embodiments.

In another example embodiment, one or more GDEs are used to inject orintroduce a depolarizing gas not only into the depolarizing electrodebut also in sufficient quantities to force the gas into the electrolyteto cause the formation of bubbles that will rise within the reactor,causing mixing within the electrolyte, and thereby increasing masstransfer and decreasing concentration polarization effects.Alternatively, one or more GDEs are used to inject an inert gas or somecombination of inert gas and depolarizing gas. In this embodiment, theGDE acts like a fine bubble diffuser, and may carry out two functions:to add a gas to the cell and also to provide mixing. Thus, thedepolarizing gas and/or an inert gas can be forced into the liquidelectrolyte, via the at least one electrode, to cause bubble formationand/or mixing in the liquid electrolyte.

In various further examples: a porous conductive material or layer isprovided attached to, positioned adjacent to, positioned or layeredupon, or at least partially within the gas permeable material; theporous conductive material or layer is associated with the gas permeablematerial; the porous conductive material or layer is located on and/orwithin the gas permeable material; and/or, the gas permeable material islocated on and/or within the porous conductive material or layer. Inother examples, the gas permeable material is a gas permeable membraneor structure, a gas permeable polymer membrane or structure, a gaspermeable porous polymer membrane or structure, or a gas permeableporous membrane or structure.

General Example Embodiments—3D Electrode and Gas Diffusion Electrode(GDE) with a Barrier Layer to Exclude Vapour from the Liquid Electrolyte

An example embodiment 3D electrode or GDE may incorporate one or morebarrier layers or barrier films that are highly or substantiallypermeable to the relevant gas stream, but relatively less permeable orimpermeable to the transport of the reaction solvent in gaseous form.Several examples of such barrier layers or films exist. Examples of suchbarrier layers or films that are highly permeable to oxygen gas butpoorly permeable or impermeable to water vapour include: polyolefins,poly(methylpentene), organosilicon polymer films, fluorocarbon orperfluorocarbon polymers, especially hyperbranched perfluorocarbonpolymers, or mixtures thereof. The incorporation of such a barrier layerin the 3D electrode, for example a 3D GDE, preserves the gas streamoutside of the 3D electrode from contamination by the gaseous form ofthe solvent used (e.g. water vapour) and also protects the gas channelsoutside of the 3D electrode from being blocked, impeded, or flooded bywater condensate. The unique properties of the 3D electrode in respectof avoiding flooding, may thereby be transmitted to the entire networkof gas channels and plumbing within a cell in which it is employed.

Additionally, because it can be practically difficult to completelyprevent the formation of larger pores in a 3D electrode or to preventdefects from forming over the course of extended use, the barrier layeror barrier film may serve as a means to mask large pores and/or defectsin the porous structure that could compromise the ability of the 3Delectrode to perform a desired function, for example such as to preventflooding.

The barrier layer or barrier film may be located on the gas side of the3D electrode. Alternatively, the barrier layer or barrier film may belocated on the liquid side of the 3D electrode, between the porousconductive material (e.g. conductive metallic material) and the gaspermeable material (e.g. non-conductive polymer layer).

Preferably, the barrier layer or barrier film is highly or substantiallypermeable to the gases that are generated (as reaction products) oradded (as reactants) from the gas side of the 3D electrode, but poorlypermeable or impermeable to the solid, liquid, or gaseous components ofthe solvent used on the liquid side of the 3D electrode, namely, theelectrolyte. For example, in 3D electrodes which form an interfacebetween liquid water and oxygen gas, the barrier layer or barrier filmis highly or substantially permeable to oxygen gas, but poorly permeableor impermeable to gaseous water vapour. In a second example in which a3D electrode forms an interface between methane/natural gas and a liquidsolvent, the barrier layer or barrier film is highly or substantiallypermeable to methane/natural gas, but impermeable or poorly permeable tothe gaseous form of the liquid solvent.

In a particular example, the 3D electrode is a composite electrode andcomprises a gas permeable material provided as a non-woven layer (e.g.high-density polyethylene fibers, such as for example Tyvek™ polymermaterial) attached to a barrier layer comprising a polymeric dense thinfilm (e.g. a polymethylpentene barrier layer) on one side, and a metalmesh on the other side, where the metal mesh is adhered to the polymerlayer by a binder material.

Some General Methods of Fabricating an Example 3D Electrode or GDE

In one example, one could start with a gas permeable material providedas a non-conductive material and then apply the porous conductivematerial by depositing a conductive metallic material on the gaspermeable material. In a further example, one or more catalysts can thenbe deposited as part of a binding layer, with subsequent lamination ofthe electrode assembly into a single structure using suitable heatand/or pressure. In a still further example, one may coat a bindermaterial to provide a binding layer containing one or more catalystsonto a gas permeable material (e.g. a polymer layer) and then laminatethe gas permeable material with a metallic material or layer pre-coatedwith the same binder material. Several other methods exist to fabricatean example embodiment.

Some General Advantages of Example 3D Electrodes and GDEs

As noted earlier, the presence of well-defined and narrowgas-solid-liquid interfaces in 3D electrodes and GDEs of the presentembodiments may eliminate many of the problems that are created in otherclasses of solid-liquid-gas electrodes, such as conventional gasdiffusion electrodes, or trickle-bed electrodes. Examples of theproblems that may be eliminated or diminished include, withoutlimitation, instability in, inhomogeneity in, fluctuations in, and/orfailure of the solid-liquid-gas boundary. Problems of this type mayresult in uneven, low yielding, incomplete or incorrect reactions,amongst others.

Additionally, the 3D electrodes/GDEs can provide unexpectedly amplifiedelectrochemical properties of the type describe earlier, includingunusually high electrode activities per unit volume of depositedcatalyst (included in the binder layer).

The inventors have found that unexpected and disproportionate advantagesof this type may be realised when the electrode interface is fabricatedin a careful, calibrated manner. For improved performance the electrodemight be operated at relatively low current densities, such as from 1mA/cm² to 500 mA/cm² or, preferably, from 1 mA/cm² to 200 mA/cm², orpreferably from 1 mA/cm² to 100 mA/cm², inclusively.

Thus, for example, hydrogen-oxygen fuel cells utilizing the 3Delectrodes typically require smaller quantities of catalysts than isnormally the case using other types of electrodes. The produced 3Delectrodes also do not necessarily require pure oxygen gas or highlycompressed atmospheric air oxygen as a feedstock (as is the case in PEMfuel cells). Nor do the produced 3D electrodes necessarily requirehumidification of the feedstock gases (as is the case in PEM fuelcells). These advantages arise because the conductive layer in 3Delectrodes of the present embodiments are well-defined, narrow, and havea high electrochemical area per unit volume of 3D electrode.

Other advantageous features which may be realised include, amongstothers: (i) the catalyst in the interfacial region is maximally active,(ii) the catalyst is not wasted by being deposited in other,non-interfacial regions, where catalyst cannot act, (iii) the reactantshave improved or maximum access to the electrode surface and sufferfewer limitations in terms of mass transport, and (iv) in one exampleapplication, water molecule products are readily and rapidly transportedaway from the reactive surface of the electrodes (due to the relativelynarrow conductive layer and its high electrochemical surface area).

For illustrative purposes only and without limiting the invention, wenow describe a representative common problem that may arise inconventional gas diffusion or particulate fixed bed electrodes and showhow it may be eliminated in a 3D electrode of the present embodiments.

“Flooding” is a phenomenon that occurs when a liquid (product orreactant) partially or completely fills a gas diffusion electrode,thereby causing a breakdown in the solid-liquid-gas boundary andblocking electrochemical contact with the gas (reactant or product).Flooding is a particular problem in fuel cells, such as hydrogen-oxygenfuel cells, that require the feedstock gases to be humidified. Floodingmay be caused by water ingress into the gas diffusion electrode viasystematic, incremental percolation through the non-homogeneous pores ofthe electrode, or it may be caused by spontaneous condensation of thewater vapour in the feedstock gas stream. Regardless of its origin,flooding always induces a decline in the voltage output and powergeneration of such fuel cells.

Flooding does not, however, occur under normal operating conditions in3D electrodes of the present embodiments since the three-phasesolid-liquid-gas boundary is too well-defined and too narrow. There is avery clear separation of the liquid and gas phases in such electrodes,meaning that incremental percolation through the GDL does not occur.Moreover, the narrowness of the interface ensures that any condensationof any size is readily taken up and drawn back into the liquid phase,thereby effectively eliminating the possibility of flooding.

The above advantages confer utility and low-cost upon 3D electrodes ofthe present embodiments, as well as high performance relative to thecurrent density employed. These properties make the 3D electrodespractical and useful in a variety of industrial applications, includingbut not limited to electro-energy and electro-synthesis applications.Many such applications are not practically viable without the use of 3Delectrodes of the present embodiments. The 3D electrodes also allow thefabrication of practical and viable devices for these transformations,such as spiral-wound reactors and the like.

In further illustrative example applications, the 3D electrodes may alsobe used to improve or make viable electrochemical devices for: (i)converting air-based oxygen into pure or purer oxygen; (ii)manufacturing hydrogen peroxide; or (iii) use as fuel cells, bothhydrogen-oxygen fuel cells and direct methane fuel cells. These exampleelectrochemical devices share a common feature in that the 3D electrodesall display unusually high electrochemical activity relative to thecurrent density employed. This activity appears to derive at least inpart, from an unexpectedly strong capacity to sequester and consumeoxygen gas from out of the air; a property that is believed to resultfrom the well-defined and narrow three-way solid-liquid-gas boundary inthe 3D electrode. The interface appears to create a remarkably selectivereaction by the oxygen in air. The reaction is so strongly favoured thatit continues within a sealed gas chamber even after the oxygen in theair has been largely depleted, thereby causing the formation of apartial vacuum in the gas chamber. Such a partial vacuum would normallyhalt or, at least, dramatically slow the reaction. However, in thesecells, the vacuum continues growing until effectively all of the oxygenin the air is consumed. To the best of the inventors' knowledge, sucheffects have not been previously observed. This was undoubtedly becausein these examples, the solid-liquid-gas boundary was carefully createdto have a width/thickness of the order of 850 nm. This meant that theelectrode could operate highly efficiently at a relatively low currentdensity.

Beyond the above, 3D electrodes of the present embodiments may alsodisplay the following advantages:

-   -   (1) A dramatically higher wetting pressure than is achievable in        any known conventional gas diffusion electrode. Conventional gas        diffusion electrodes typically flood upon the application of        <0.2 bar of external pressure. By contrast, electrodes of the        current embodiments contain uniform pore structures in the gas        permeable, water impermeable layers, so that they may require        far higher external pressures before leaking. For example,        embodiment electrodes may contain relatively small/tiny and        uniform pore sizes, such as from about 10 nm to about 500 nm, or        in one example about 0.2 microns, which can lead to a reduction        in or avoidance of flooding of the electrode up to applied        pressures of 3.4 bar. This means that a substantial pressure        differential can be applied across the electrode, e.g. having an        electrolyte at higher pressure on one side of the electrode        compared to a gas region on the other side of the electrode, for        example a pressure difference of greater than or equal to about        3.4 bar, well above previously known electrodes. By this means,        electrodes of the present embodiments can withstand a relatively        higher pressure before leaking.    -   (2) Flexibility of the electrode; the materials used in the        electrode can be optionally made to be flexible or bendable, and        for example, able to be rolled or spiral-wound. The gas        permeable material can be selected from, for example, different        porous polymer materials and/or different pore sizes to achieve        desired properties of the electrode. This flexibility        distinguishes many previous electrodes that are rigid        structures.    -   (3) The ability to produce electrodes of relatively large size.        For example, for commercial applications, electrodes can be        readily produced having a width and/or a length of greater than        or equal to 0.05 m, 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 1 m, or        2 m. In another example electrodes can be readily produced of        about 0.05 m, about 0.1 m, about 0.2 m, about 0.3 m, about 0.4        m, about 0.5 m, about 1 m, about 2 m, or larger in width and/or        length. In an application where an electrode is rolled or        spiral-wound, the flat electrode before rolling may preferably        have a width of about 0.05 m or greater, about 0.1 m or greater,        about 0.2 m or greater, about 0.3 m or greater, about 0.4 m or        greater, about 0.5 m or greater, about 1 m or greater, about 2 m        or greater, and a length of about 0.5 m or greater, about 1 m or        greater, about 2 m or greater, about 3 m or greater, about 4 m        or greater, about 5 m or greater, about 10 m or greater. The        rolled or wound electrode may have a diameter of about 0.05 m or        greater, about 0.1 m or greater, about 0.2 m or greater, about        0.3 m or greater, about 0.4 m or greater, about 0.5 m or        greater, or even larger. This relatively large size        distinguishes many previous electrodes that can only be produced        in a small size, for example up to the order of 0.01 m in size.        The difference in size scale is not a trivial factor since many        small electrodes cannot be simply scaled up in size. For        example, in relatively small cells having small sized        electrodes, it is not required to have or consider a high        electrical conductivity in the cell/electrode, because the        distances involved are small, so the associated resistances are        relatively small. In contrast, in larger scale cells/electrodes,        such as the present example, this issue is much more challenging        and higher conductivity is required along very good conduction        pathways. Hence, a small scale electrode structure cannot        typically and simply be scaled up to a large scale electrode.

Further Aspects of Example Gas Diffusion Electrodes (GDEs)

For the purposes of this illustrative example, we refer to thecombination of an expanded PTFE (ePTFE) membrane (General ElectricCompany: pore size 0.2 micron) (i.e. a gas permeable material) overlaidwith a fine nickel mesh (200 lines per inch; manufactured by PrecisioneForming Inc.) (i.e. a porous conductive material), optionally heldtogether by a binder material, or a binder-catalyst material, includingabout 5-15% Nafion in alcohol/water (supplied by Ion Power Inc.), andabout 20-50% by weight of fillers and/or catalyst material.

FIG. 1 depicts in a schematic form, a conventional gas diffusionelectrode (GDE) 110, as widely used in industry at present (Prior Art).In cases where an electrode contains a zone or a layer that is intendedto facilitate gas diffusion, FIG. 1 illustrates that gas diffusion layeror zone. FIG. 2 illustrates in schematic form the general structure ofan example 3D electrode 115. In a conventional GDE 110, conductiveparticles (such as carbon particles) are typically mixed withnon-conductive, hydrophobic Teflon particles, and then compressed and/orsintered into a single unit whose pore structure is ill-defined andnon-uniform. By contrast, in an embodiment of the present GDE 115, theporous conductive material 130 and the gas permeable material 120 aresubstantially demarcated or separated, although there can be overlap ata boundary region. The pore structure of the gas permeable material 120,for example a non-conductive, hydrophobic material/element, iswell-defined and uniform.

As can be seen in FIG. 3, the example 3D electrode (or GDE) 205 of widthw includes a conductive layer or region 210 of width d with anon-conductive layer or region of width w-d. The dimensions are notaccurate and are for illustration only. In the case of one particularexample of a laminated electrode, the 3D conductive layer 210 (i.e.porous conductive material) comprises fine nickel mesh, which has athickness of about 5-8 μm, while the 3D non-conductive layer 211comprises an ePTFE membrane, which has a thickness of about 20 μm.

While the non-conductive layer or region 211 is thicker than theconductive layer or region 210 in this case, that need not be true inother cases of fabricated 3D electrodes. With other gas permeablematerials and other techniques, this relative ratio may be quitedifferent, with conductive layers or regions 210 being thicker and thenon-conductive layers or regions 211 being thinner.

For example, in the case of an electrode where a binder material wasapplied with a paintbrush, the conductive layer comprised the finenickel mesh and the binder material. The thickness of the bindermaterial providing a binding layer was not easily controlled using apaintbrush, so that thicknesses of a binding layer of up to about 112μm, for example, may be created. The binder material, moreover,penetrated the outermost portion of the ePTFE layer or membrane (toabout 0.1-0.8 μm deep), so that the conductive portion may becumulatively up to about 120 μm in thickness. The non-conductive portionwould typically be about 19.2-19.8 μm thick. Thus, in such a case, thethree-phase solid-liquid-gas boundary will fall within a maximumthickness of 0.8+120=120.8 μm. Such large thicknesses generallyrepresent an extreme in the case of GDEs of the present embodiments,although thicknesses of 400-500 μm have also been achieved in the mostextreme cases. Generally, but not exclusively, GDEs of the presentembodiments formed by lamination of free-standing porous metallicstructures to ePTFE membranes will have a three-phase solid-liquid-gasboundary that is less than about 100 μm thick.

In conventional GDEs, the entire GDE is conductive and different poresizes and intermediate amounts of Teflon binder within the GDE, are usedto create the solid-liquid-gas boundary that is formed inside theconventional GDE. However, because the pores in conventional GDEs arecreated by fusing layers of different particle size, there is relativelypoor control on the pore size and distribution. The pores are thereforeof a generally wide and non-uniform distribution. Moreover, the poresare generally large, being, at best, typically 30 microns in diameter atthe outside edges of the GDE. The solid-liquid-gas boundary that iscreated within the GDE is therefore poorly controlled and ill-defined,with substantial variations in depth within the GDE. Small changes thatoccur during use of the GDE may therefore also shift the interface,causing the GDE to be prone to instability or even breakdown. Thus, acommon problem in gas-liquid electrochemical transformations is floodingof the GDE. This occurs when the solid-liquid-gas boundary progressivelyrelocates itself into the center of the GDE, until the GDE iseffectively filled with liquid.

Whereas a conventional GDE relies upon the presence of larger pores inthe center to provide for low-pressure ingress of gases to theinterface, example GDEs of the present embodiments rely upon asubstantial, large, relatively large or substantially largenon-conductive layer or region 211 relative to the volume of theinterface 235 with the conductive layer or region 210, to provide forlow-pressure ingress of gases.

One advantage involves hitherto unavailable uniformity in howelectrochemical gas-liquid reactions take place down the full length ofthe 3D GDE. Because the solid-liquid-gas boundary is so tightlyconstrained and uniform, such reactions will essentially occur in anidentical way at all points of the interface along the length of theelectrode. Practical problems arising from inhomogeneity and instabilityin the interface, as occur in many conventional GDEs, may therefore belargely eliminated. These include, without limitation, local excesses(or swamping/flooding) of reactants/products, leading to inefficientreaction, energy wastage (e.g. local hotspots), or electrodedegradation. Moreover, once created, the interface is relatively stableand easily maintained—more stable and easily maintained thatconventional GDEs. These properties result in 3D electrodes that may bemore active per unit electrochemical area or per unit volume of catalystthan comparable conventional GDEs.

Another feature is that the solid-liquid-gas boundary is relativelydelicate. By this it is meant that the solid-liquid-gas boundary can bedegraded (reversibly and temporarily) by non-judicious applications ofgas pressure (from the gas-facing side). For example, even relativelysmall overpressures on the gas side of the GDE can push the liquid outof the conductive layer, diminishing the surface area of the boundary.This will occur at the so-called “bubble point” of the membrane polymerlayer. In the extreme case, the liquid may even be pushed away from theelectrode, effectively, destroying the solid-liquid-gas boundary ormaking it so small as to be practically useless. Moreover, in such asituation, gas bubbles may become trapped in the conductive layer orregion 210, making it difficult (but not impossible) to regenerate theelectrode. To avoid these possibilities, it is generally desirable ornecessary to closely control external gas pressures and ensure that theconductive layer or region 210 is properly “wetted” prior to operation.Once operating, GDEs of the present embodiments are generally highlystable. While the solid-liquid-gas boundaries are “delicate” in thatthey may be destroyed or disrupted upon the application of excesses ofpressure, it should be noted that the pressures required to disrupt thethree-phase boundaries are much higher than is the case in conventionalGDEs. That is, the three-phase solid-liquid-gas boundaries in exampleGDEs are much less delicate than is the case for conventional GDEs.

Considering another aspect of example electrodes, there are various waysto measure air permeability of a material. For example, porosimietry canbe used to determine the flow rate of air through membranes and coatedmembranes in units of liters per minute (L/min) as a function of appliedpressure (in units of psi). Another way to measure air permeability isto use the ‘Gurley number’ scale, which is obtained with a Gurleydensitometer. This measures the time (in seconds) taken to pass aparticular fixed volume of air (2.5 cm³) through a sample of fixed area(0.645 cm²) at a fixed applied pressure (0.44 psi). The air permeabilityof the sample is inversely proportional to the Gurley number. That is,the larger the Gurley number, the less permeable to air is the sample.

Present example 3D electrodes/GDEs, for example using a treated orcoated ePTFE membrane, have an air permeability that is very similar tothat of the untreated or uncoated ePTFE membrane, at all measuredapplied pressures. By contrast, the air permeability of a conventionalgas diffusion electrode using a Gortex™ membrane as an ‘electrolyteleakage barrier’ declines very substantially. For example, Gortex™membranes used in conventional gas diffusion electrodes typically haveGurley numbers of 50-800 seconds. In one example, after they werelaminated to a conventional gas diffusion electrode, their Gurley numberincreased to about 9,000-16,000 seconds. This means that it took 20-180times longer to transfer the same quantity of air through such anelectrode (with a Gortex™ membrane) as it took to transfer the samequantity of air through the Gortex™ membrane only.

Thus, in some particular example 3D electrodes according to presentembodiments, an advantage is that the 3D electrodes have improvedpermeability to air, or are substantially permeable to air, whereasconventional 3D electrodes are less so. That is, in one example, the airpermeability of the 3D electrode is similar to, about equal to, the sameas, or is substantially similar to, substantially about equal to, orsubstantially the same as, the air permeability of the gas permeablematerial (e.g. polymer membrane).

FIG. 4 schematically illustrates a GDE 208 in which a gas permeablematerial, such as a gas permeable polymer layer, has been laminatedwith, or attached to a porous conductive material, such as a conductivemetallic layer, on both of its sides. The second conductive layer 250may be applied to the GDE 208 at the same time as the first conductivelayer 220. Alternatively the second conductive layer 250 may be appliedafter the first conductive layer 220 is applied. The same means offabrication described in the earlier examples, or other means, may beused to generate the double-sided GDE 208.

Regardless of its method of manufacture, the effect of having metalliclayers, regions or coatings on both sides of the GDE 208 is to make thecentral, non-conductive core or region 211, also a channel along whichgases can pass. The outer metallic layers, regions or coatings face theliquid phase (e.g. in one example water).

The resulting membranous gas channel 211 within the body of such adouble-sided gas diffusion electrode 208 may be remarkably permeable togases. That is, the resulting gas channel may be able to accommodate andcarry unexpectedly large quantities of gas even at atmospheric pressure.For example, in a particular but non-limiting application, when actingas a cathode in a water electrolysis cell operating at a current densityof about 10 mA/cm² (which results in the generation of 1000 litres ofhydrogen per day per square meter of electrode geometric surface), sucha double-sided gas diffusion electrode 208 can extend up to about 2.5meters away from an attached hydrogen collection apparatus without theinner gas channel of the electrode 208 becoming saturated and unable tocarry more hydrogen at any point along its length. Such a double-sidedGDE 208 may be used by dipping into a solution of electrolyte, with gasfed to or from the non-conductive central region or core 211.

Novel Properties of Example Gas Diffusion Electrodes (GDEs)—the Effectof Pressure and Temperature on Energy Efficiency and Flooding

A feature of example GDEs of the present embodiments is that they allowfor the application of a higher pressure to the liquid electrolyte thanis present on the gases in the GDE. High liquid pressures (relative tothe corresponding pressure of the gas on the gas-facing side of the GDE)often have the effect of improving the energy efficiency ofelectrochemical reactions. By contrast, conventional GDEs typically canonly deal with very low liquid pressures before they flood (and therebybecome inoperable).

For example, GDEs containing as their polymer layer, a General ElectricCompany PTFE membrane with average pore size 0.2 μm (used for membranedistillation in the water purification industry), are typically able towithstand up to about 3.4 bar of liquid pressure before they flood. Thisis because the PTFE membrane has a wetting pressure (or “water-inlet”pressure) of 3.4 bar.

Thus, an electrochemical cell employing such GDEs may have its liquidelectrolyte pressurised up to 3.4 bar higher, in this case, than thepressure of the gases in and on the gas-facing sides of the GDEs. Manyelectrochemical processes involving gas-to-liquid or liquid-to-gastransformations are favourably affected by differential pressures ofthis type. Such a large pressure differential may therefore have theeffect of substantially increasing the energy efficiency of thehalf-reaction which occurs at the GDE electrode. That is, one mayachieve a particular rate of production at a lower applied cell voltagethan was otherwise needed.

By contrast, conventional GDEs have wetting pressures that are said notto exceed 0.2 bar, meaning that they readily allow electrolyte to leakeven at very mild liquid pressures. The option to apply higherdifferential pressures above 0.2 bar to liquid electrolytes in suchcases, is therefore not available.

Thus, in one example embodiment, an electrochemical cell employing a GDEcan have its liquid electrolyte pressurised to at least 0.2 bar and upto about 3.4 bar higher than the pressure of the gases in and on thegas-facing sides of the GDE.

A second feature of example GDEs of the present embodiments is theirunusual properties at increasing temperatures. One effect of highertemperatures is to increase the amount of water vapour within a GDE andtherefore also to increase the potential for condensation of that watervapour (flooding) within the GDE. An example GDE, with a high wettingpressure of, for example, 3.4 bar, is far less easily wet (if not being,effectively un-wettable) than a conventional GDE with a wetting pressureof 0.1 bar. For this reason, the conventional GDE will be at greaterrisk of flooding with increasing temperature than a GDE of the presentembodiments with a higher wetting pressure (e.g. 3.4 bar).

Thus, cells employing example GDEs of the present embodiments may havetheir liquid electrolyte heated to higher temperatures than those havingconventional GDEs, without risk of flooding the GDE. For manyelectrochemical processes, higher temperatures have the effect ofimproving the energy efficiency of the half-reaction at the electrodeand thereby the increasing the energy efficiency of the overall process.Moreover, most electrolytic cells are “self-heating” in that the excessenergy which must be applied to drive the reaction, is released as heat.

Example 1 Fabricating GDEs Using Deposition of Conductive Metals

In other examples there are provided 3D electrodes/GDEs which include agas permeable material and a porous conductive material partiallycoating the gas permeable material. Referring back to FIG. 3 toillustrate this electrode structure, the porous conductive materialpenetrates the gas permeable material to a depth (d) less than thethickness (w) of the gas permeable material. For example, the depth isbetween about 5 nanometers to about 0.4 millimeters, dependent onsufficient thickness of the gas permeable material, e.g. gas permeablemembrane. Alternatively, in another preferred form, the depth is betweenabout 1/100,000^(th) to about 1/1.5^(th) of the thickness of the gaspermeable material.

A conductive layer is formed at one surface of the 3D electrode and anon-conductive layer is provided or formed at the other, opposing,surface of the 3D electrode/GDE. The conductivity of the 3D electrodethus varies along the thickness of the 3D electrode. The conductivelayer is gas permeable and at least partially liquid permeable, whereasthe non-conductive layer is gas permeable and liquid impermeable, duringnormal operational use of the electrode. The conductive layer is part ofan outer surface of the 3D electrode and is relatively less hydrophobicthan the gas permeable material, whereas the bulk 3D electrode is gasbreathable and liquid impermeable.

In other example forms: when used as a GDE, a three-way solid-liquid-gasboundary is formed within the 3D electrode; the solid-liquid-gasboundary is narrow in macroscopic width compared to the thickness of the3D electrode or the gas permeable material. For example, thesolid-liquid-gas boundary may be up to 850 nm wide.

Generally, for the examples discussed here, there is provided a processfor preparing a 3D electrode or a GDE, comprising the steps of: afabrication step to fabricate the 3D electrode or a GDE, includingdetermining or setting a width of a three-phase solid-liquid-gasboundary, preferably where the width is narrow in relation to the widthof the 3D electrode or a GDE; and an operation step to operate the 3Delectrode or a GDE, preferably in a cell, at low current density, forexample from 1 mA/cm² to 500 mA/cm², or from 1 mA/cm² to 200 mA/cm², orfrom 1 mA/cm² to 100 mA/cm², inclusively.

Referring back to FIG. 3 as a structural illustration for thisalternative example, where the metallic and/or binder material haspenetrated the pores of the non-conductive layer or region 211 theconductive layer or region 210 closest to the interface 235 or boundaryregion may also have a pore structure and other properties (e.g.hydrophobicity, wetting, and gas/liquid permeability), that areessentially identical, or, at least, very similar, to that of thenon-conductive layer or region 211. In such a case, the boundary regionor interface 235 between the layers or regions 210, 211 is not so muchcharacterised by a structural change, as by an electrical change. It is,effectively, only a boundary region or interface of electricalconductivity. On one side of boundary or interface 235, layer or region210 is conductive or somewhat conductive, whereas on the other side ofboundary or interface 235, layer or region 211 is non-conductive.Moreover, on both sides of the boundary, boundary region or interface235, the pores are uniform and small (about 0.2 micron in this case,although smaller pores can be obtained using other membranes). For thistype of example 3D electrode, there is a substantially uniform or highlyuniform pore structure and distribution, especially about theconductive-non-conductive boundary, which can be readily varied bymerely selecting a different membrane to use as a gas permeablematerial. Important other properties (e.g. hydrophobicity, wetting, andgas/liquid permeability) are also unchanged on both sides of theinterface 235.

The gas permeability of the conductive layer or region 210 is, moreover,either identical to or greater than that of the non-conductive layer orregion 211 (except, of course, in the non-optimum case where themembrane has been blocked by an over-thick application of the conductivelayer). Thus, gases may readily and uniformly pass through the electrode205 (in this alternative example). The gas permeability of the 3Delectrode 205 is, additionally, readily characterizable, being createdby and being substantially the same as that of the uncoated gaspermeable material, for which gas permeability data may routinely exist.

The liquid permeability of a 3D electrode depends largely or evenentirely on the gas permeable material and the liquid with which itinteracts. A hydrophilic polymer allows a hydrophilic liquid to passthrough evenly and uniformly. The same is true for a hydrophobic polymerinteracting with a hydrophobic liquid. In the case where there is amismatch between the polymer and the liquid, an interface is createdbetween the liquid and the 3D electrode. The extent and nature of thatinterface depends on the materials involved.

In further various examples, the wetting pressure for the GDEs is thesame as that of the polymer layer or membrane used (for example theGeneral Electric Company membrane of 0.2 μm average pore size), which isabout 3.4 bar. Thus, only upon the application of 3.4 bar of pressure onthe liquid side does liquid water penetrate and pass through themembrane, thereby flooding the membrane. By contrast, all other GDEsknown to the inventors have wetting pressures that do not exceed 0.2bar.

In various further examples: a porous conductive material or layer isprovided at least partially within the gas permeable material; theporous conductive material or layer is associated with the gas permeablematerial; the porous conductive material or layer is located on andwithin the gas permeable material; and/or, the gas permeable material islocated on and within the porous conductive material or layer.Preferably, though not necessarily, the conductive material is a metal,which after being applied is in the form of the porous conductivematerial. For example, the conductive material forming the porousconductive material can be Nickel. Alternatively, the metal could be Ti,Cr, Pt, Cu, Pb, Sn. Co, Mn, Au or Ag. Further, the porous conductivematerial could be formed of carbon black particles.

In further examples, the depth (d) of the conductive layer or portion isin the range of about 1 nm to about 10 μm, or about 50 nm to about 1 μm,or about 50 nm to about 500 nm. In a specific example, the porousconductive material is formed of Nickel, the gas permeable material is a0.2 micron PTFE membrane and the depth is greater than 0 and less thanabout 850 nm.

In an example method of fabricating this form of 3D electrode, the stepsinclude selecting a gas permeable material, for example with asubstantially uniform pore size, and then applying, as a calibratedstep, a conductive material to partially coat the gas permeablematerial, thereby forming a porous conductive material. The porousconductive material penetrates the gas permeable material to a depthless than the thickness of the gas permeable material. The calibratedstep can include: a particular mode of application, a particular time orperiod of application; a particular electrical current or range ofcurrent of application; a particular temperature or range of temperatureof application; and/or a particular circumstance or range ofcircumstances of application. The ideal conditions by which thecalibrated deposition is performed, are typically determined by aprogram of study to realise a suitably narrow and well-definedsolid-liquid-gas boundary in the desired range of widths, such as from50 to 850 nm width. In one example, the conductive material can beNickel and can be applied by vacuum deposition at a loading of greaterthan about 0.455 g/m² and less than about 3.64 g/m². Preferably, in thisparticular example, the Nickel is provided at a loading of about 1.82g/m², which has the effect of imparting the electrode with unexpectedlyamplified electrochemical properties when operated at a current densityof 10 mA/cm² in the manufacture of: (i) pure oxygen from air oxygen,(ii) hydrogen peroxide from aqueous alkaline solution, or (iii)electrical potential and current in an alkaline fuel cell (or a directmethane fuel cell when a coating of Pt is used having 100 nm thickness).

Calibrated or careful application of one or more electrically conductivematerials to gas permeable materials, for example porous polymermembranes, using controllable coating techniques can be used to produce3D electrodes. When formed in a calibrated manner, one or moreconductive layers may form part of a 3D electrode with unexpected andimproved electrochemical performance, especially relative to other 3Delectrodes and to the cost of manufacture. Further layers, for exampleincluding catalytic or other materials, can be conveniently added to, orformed upon the one or more conductive layers to produce more complex 3Delectrodes that are practical and useful in electro-energy orelectro-synthetic applications.

Example gas permeable or breathable 3D electrodes can be formed bydepositing a conductive material or layer on a gas permeable materialand, optionally, subsequently depositing a catalyst on the conductivematerial or layer. In one example, one could start with a gas permeablenon-conductive material and then form the conductive material or layeron the gas permeable non-conductive material, and thereafter, depositone or more catalysts.

In the case of an example 3D electrode manufactured in this manner, andreferring back to the structure illustrated in FIG. 3, a gradual changein hydrophobicity exists in moving from the outside surface 220 throughthe conductive layer or region 210 which may penetrate the gas permeablematerial to depth d. The outer metal-binder surface 220 is relativelyless hydrophobic, but this becomes more hydrophobic on moving into thenon-conductive layer or region 211 toward the highly hydrophobic,non-conductive surface 230. The distance over which this hydrophobicitychanges may be small, in one example being effectively only the depthinto which the binder material penetrates the gas permeable material,for example in the case of ePTFE pore structure about 0.1-0.8 μm. Thisis narrower than the depth d, which defines or approximates thethickness of the conducting layer (for example about 8 pin to about 120μm in some examples).

Thus, for this particular 3D electrode, a liquid solvent like water islikely able to partially penetrate at least some of the way into theconductive outer layer or region 210, which in one example form may beprovided by applying or depositing a metallic coating. But water will berepelled and unable to penetrate into the highly hydrophobic interior.The liquid is therefore limited to, in one example the about 0.1 μm toabout 0.8 μm thick outermost portion of the ePTFE, which has a highinternal surface area, most of which may be conductive (after attachmentof the metallic coating). The ingress of liquid water into the electrode205 is therefore tightly controlled and a solid-liquid-gas boundary iscreated within, in one example, the outermost layer of about 0.1 μm toabout 0.8 μm in depth. At this interface, gas from the non-conductiveside 230 of the electrode 205 encounters liquid ingression from theoutside of the membrane, at the conductive, metallized region.

According to various aspects provided by way of example:

-   -   (1) Carefully calibrated application of one or more conductive        materials to gas permeable materials, such as porous polymer        membranes, using controllable coating techniques can produce 3D        conductive electrodes of remarkable and unexpected robustness,        activity, and electrochemical area per unit volume, and which,        when configured for gas-to-liquid and/or liquid-to-gas        processes, display uniquely well-defined, narrow, and stable        three-way solid-liquid-gas boundaries;    -   (2) When applied in a calibrated manner, conductive layers of        this type constitute the formation of a 3D electrode with        unexpected and amplified electrochemical performance, especially        relative to other 3D electrodes and to the cost of manufacture;    -   (3) Additional layers including catalytic or other materials may        be conveniently added to, or formed upon the conductive one or        more layers to yield more complex 3D electrode structures that        are practically useful in, especially, electro-energy or        electro-synthetic applications;    -   (4) The availability of 3D electrodes, for example fabricated as        described in points (1)-(3) above, makes viable or improves the        practicality of a range of electro-energy and electro-synthetic        applications. Such applications are typically unviable or        relatively less practical using conventional fixed-bed or gas        diffusion electrode technologies.

In various example forms, the coating techniques include but are notlimited to metal vacuum-coating, sputter-coating, dip-coating,electroless- and electro-coating, powder-coating, and the like. Invarious example forms, the catalytic or other layers are applied bytechniques, including but not limited to: electro- orelectroless-coating, powder-coating, dip-coating, vacuum-coating, andthe like. While coating techniques such as these have been previouslyapplied to membranes which have subsequently been used to facilitateelectrocatalytic transformations, the inventors have found that suchmetal-coating can be optimised in a different way, which provides fornovel and improved catalytic properties, especially, but notexclusively, when operated at low current density. The unique mode ofoptimisation in such cases is directed at achieving a well-defined andnarrow solid-liquid-gas boundary during operation as a GDE, such ashaving a macroscopic or maximum width of from about 50 to about 850 nm.Optionally, the metal-coated materials or membranes may be furthercoated with other particulate materials, slurries or coatings, includingbut not limited to materials that may comprise, wholly or in part, ofelectrochemically active ingredients. Such electrochemically activeingredients may include but are but not limited to: (a)electrocatalysts, (b) conductors, (c) current collectors, and the like.

Optionally, but preferably, the 3D electrode is flexible. Optionally,but preferably, the gas permeable material is made at least partially orwholly from a substance that is flexible, for example at least partiallyor wholly from a polymer or a combination of polymers, for example PTFE,ePTFE, polyethylene, polysulfone or polypropylene. The polymer itselfmay or may not be gas permeable. For example, the polymer itself may notbe gas permeable but a structure or membrane formed from the polymer isgas permeable.

Example 2 Fabricating GDEs Using Lamination

In another specific example, an expanded PTFE (ePTFE) membranemanufactured by General Electric Company for the water treatmentindustry (pore size 0.2 micron) had a fine nickel mesh (200 line perinch; manufactured by Precision eForming Inc.) laid down upon themembrane. The mesh was then carefully lifted, starting at one edge and alayer of a binder material (15% Nafion in alcohol/water; supplied by IonPower Inc., containing 10% by weight of carbon black, supplied bySigma-Aldrich) was applied to the membrane surface. The mesh wasthereafter released and allowed to contact the coated membrane. Afterleaving to dry for 4 hours at 60° C., the mesh was adhered to thesurface of the PTFE membrane. This fabrication method may be amended inseveral ways. The binder material may be applied or painted over theunconnected mesh and the membrane and then dried, causing the mesh toadhere to the membrane. Alternatively, the binder material may beseparately applied to the membrane surface and the mesh, with thecoated, wet membrane and mesh then married up and dried.

Further aspects and details of example electrodes that can be utilisedas GDEs can be found in the Applicant's concurrently filed PCT patentapplication “Composite Three-Dimensional Electrodes and Methods ofFabrication” filed on 30 Jul. 2014, which is incorporated herein byreference.

Example 3 Deploying Example Embodiment GDEs in Industrial Applications

The 3D electrodes being applied as GDEs allows a new type ofelectro-synthetic (i.e. electrochemical) or electro-energy cell, e.g.fuel cell, to be achieved. The cell includes a liquid electrolyte and atleast one gas diffusion electrode as discussed above. The GDE in use canoperate as a gas depolarized electrode and includes a gas permeablematerial that is substantially impermeable to the liquid electrolyte,during normal operational use of the electrode, as well as a porousconductive material provided on a liquid electrolyte-facing side of theat least one gas diffusion electrode. The porous conductive material canbe attached to the gas permeable material by being laminated to the gaspermeable material. Alternatively, the porous conductive material isattached to the gas permeable material by being coated on at least partof the gas permeable material.

The GDE and the materials or layers used to form the GDE are optionally,but preferably, flexible. This advantageously allows the GDE, andreactors or cells which include the GDE, to be bent and wound. In orderto form spiral wound devices, a multi-layered arrangement of flat-sheetmembranes may be rolled up into a spiral wound arrangement. The spiralwound arrangement may then be encased in a casing, which holds thespiral-wound element in place within a module whilst allowing forelectrolyte to transit through the module. Alternatively and optionally,the multi-layered electrochemical reactor in a flat-sheet arrangement isnot wound into a spiral, but deployed in its flat-sheet arrangement;that is the electrochemical reactor is a flat layered arrangement. Anadvantage of this cell arrangement is that it provides for high densityof construction and may thereby provide an inexpensive way of deployinggas diffusion electrodes in an electrochemical reactor or cell.

In another embodiment there is provided an electrochemical reactor,comprising a plurality of hollow fibre electrodes (as either or both ofcathode or anode) and a plurality of other electrodes (as the oppositeelectrode). A plurality of hollow fibre cathodes comprise a hollow fibregas permeable, but electrolyte-impermeable material having a conductivelayer, that may include a catalyst. A plurality of hollow fibre anodescomprise a hollow fibre gas permeable membrane having a conductive layerthat may include a catalyst.

Example 4 Utilising GDEs with Wetting Pressures/Bubble Points Above 0.2Bar

GDEs of this type or class may be very useful in industrialelectrochemical reactions when embodiments of the method and/orelectrochemical cell are applied. The resulting improvement in energyefficiency or other benefits that are typically realised originate intwo key features which must be created and maintained in GDEs in orderto achieve maximal efficacy:

-   -   a. The three-way solid-liquid-gas interface within the GDE        should be maintained in a well-defined, narrow, and/or stable        state during operation. The higher the quality of this interface        and its reproducibility, the more electrochemically and        catalytically active the GDE is likely to be. This is because        gas-liquid reactions depend critically on a clear and invariant        interface.    -   b. The electrode face of the GDE should be maintained as        bubble-free or substantially free of new bubble formation,        during operation. This is because bubbles at the electrode        surface hinder reactants from reaching the surface and products        from departing from the surface (the bubbles “mask” the        electrode surface). Additionally, bubbles displace electrolyte        from between the electrodes (i.e. they replace electrolyte with        gaseous voids). This has the effect of potentially greatly        increasing the solution resistance, resulting in wasteful energy        consumption.

Embodiments of the method and/or electrochemical cell help to improve,create and/or maintain the above features, as best possible for the GDEsused. For illustrative purposes only, we describe examples of somerepresentative case where the method and/or electrochemical cell helpsto create and maintain the above features in a GDE with a wettingpressure and/or bubble point of more than 0.2 bar.

In one example, the method and electrochemical cell may help maintainthe quality of the three-way solid-liquid-gas interface, whilst stillcreating conditions that are maximally advantageous for the reactionitself. Thus, consider a reaction which is most advantageously carriedout at very high absolute gas pressure. Normally it would be extremelydifficult to apply a very high gas pressure through a GDE whilst stillmaintaining the gas-liquid interface. However, example embodiments allowfor high or extremely high gas pressures, by providing that the liquidphase is pressurised such that the differential pressure of the gasphase over the liquid phase does not reach the bubble point. In thisway, the quality of the gas-liquid interface is maintained and, indeed,provides a means to create and maintain the gas-liquid interface even athigh or very high applied gas pressures.

In another example, the method and/or electrochemical cell helpssuppress bubble formation at the GDE for the case of an electrochemicalprocess where a large differential pressure of the liquid side over thegas side, is preferred or optimum. This may arise when a reactantchemical species in the liquid electrolyte is transformedelectrochemically into a gaseous product at the electrode surface of theGDE. In such a case, a high pressure differential of the liquid sideover the gas side will typically have the effect of increasing thethreshold partial pressure of the gas at the electrode surface requiredto create and hold up a bubble in the liquid electrolyte. This thresholdpartial pressure will, theoretically, be increased by the same amount asthe differential pressure. For example, consider the situation where, atatmospheric pressure, bubbles are formed in the liquid electrolyte atthe conductive surface of a GDE when the gas partial pressure at thatsurface reaches 5 bar. Now consider the situation where 2 bar ofpressure is applied to the liquid phase, while the gas phase ismaintained at atmospheric pressure. In order to form bubbles at theelectrode surface, the gas partial pressure at the surface would nowhave to be more than 7 bar (=5 bar normally+2 bar additional appliedpressure). In making bubble formation more difficult, the product gas isthereby instead encouraged to migrate directly from the electrodesurface through the gas permeable, liquid-impermeable portion of the GDEto its gas-facing side.

The method and/or electrochemical cell may similarly help suppressbubble formation at the GDE in an electrochemical process where areactant gas is transformed into a liquid-product at the GDE. In thiscase, the reactant gas migrates from the gas side through the GDE to itselectrically conductive surface to there be transformed into theliquid-phase product. In such cases, bubbles are formed at the electrodesurface only when the gas pressure exceeds the so-called “bubble point”of the GDE. The effect of increasing the pressure on the liquid side ofthe GDE is then, effectively, also to increase the bubble point by thesame amount and thereby make bubble formation less likely. For example,the bubble point of the above-cited GDE utilizing expanded PTFE (ePTFE)membrane with 0.2 μm pores, is in the region of 2 bar. Thus, if, duringa sudden and unexpected gas pressure swing, the GDE gas pressure were toreach 2 bar while the GDE liquid pressure was atmospheric, bubbles willform at the electrode surface. However, if the liquid was pressurised to3 bar, then bubbles will form at the electrode surface only if the GDEgas pressure were to unexpectedly reach 5 bar (=2 bar normal bubblepoint+3 bar additional applied pressure). Thus, high pressures on theliquid electrolyte relative to the gas side of the GDE may discourageand suppress bubble formation in this case also.

In avoiding or suppressing bubble formation in one of the above ways,one may therefore:

-   -   (i) increase the inherent efficiency of the liquid-to-gas        chemical transformation, and/or    -   (ii) minimize the negative effects that are typically associated        by the presence of bubbles at electrode surfaces in        electrochemical cells.

For example, GDEs may be conveniently and reliably fabricated with tiny(less than about 500 nm, or less than about 250 nm) and uniform poresthat are not easily or inexpensively achieved in the fabrication ofconventional GDEs. For example, the average pore size can be from about50 nm to about 500 nm, or from about 100 nm to about 500 nm, or fromabout 100 nm to about 250 nm, or in more specific examples about 0.1,0.2, 0.3, 0.4 or 0.5 microns. Additionally, the desired pore size andother properties can be readily varied by simply selecting a differentpolymer membrane for adaption into a GDE. Membranes with a wide varietyof pore sizes and uniformly-distributed physical properties are readilyavailable. By controlling the pore size of the substrate polymer, onemay also control important physical properties of the GDE, such as thewetting pressure, the bubble point, and its permeability to gases.

GDEs of this class or type typically have substantially the same wettingpressure as that of the gas permeable polymer membrane substrate used.For example, a PTFE membrane (available from General Electric Companyfor membrane based distillation) having average pore size 0.2 μm has awetting pressure of 3.4 bar. A GDE containing such a membrane as thenon-conductive, gas permeable, polymer layer (the gas permeablematerial), next to or on which the metallic material, element or coating(the porous conductive material) is located, will typically also displaya wetting pressure of about 3.4 bar. Thus, liquid water will onlypenetrate and flood the GDE upon the application of 3.4 bar of pressureby, or on the liquid. Moreover, PTFE is resistant to, and unaffected bycaustic solutions, such as the 32% NaOH solutions used at the cathode inchlor-alkali cells. Metallic elements laminated with, attached to orcoated on the PTFE membranes, such as nickel or nickel meshes, are alsoresistant to and unaffected by caustic solutions.

The ability to produce electrodes of relatively large size. For example,for commercial applications, electrodes can be readily produced having awidth and/or a length of greater than or equal to 0.05 m, 0.1 m, 0.2 m,0.3 m, 0.4 m, 0.5 m, 1 m, or 2 m. In another example electrodes can bereadily produced of about 0.05 m, about 0.1 m, about 0.2 m, about 0.3 m,about 0.4 m, about 0.5 m, about 1 m, about 2 m, or larger in widthand/or length. In an application where an electrode is rolled orspiral-wound, the flat electrode before rolling may preferably have awidth of about 0.05 m or greater, about 0.1 m or greater, about 0.2 m orgreater, about 0.3 m or greater, about 0.4 m or greater, about 0.5 m orgreater, about 1 m or greater, about 2 m or greater, and a length ofabout 0.5 m or greater, about 1 m or greater, about 2 m or greater,about 3 m or greater, about 4 m or greater, about 5 m or greater, about10 m or greater. The rolled or wound electrode may have a diameter ofabout 0.05 m or greater, about 0.1 m or greater, about 0.2 m or greater,about 0.3 m or greater, about 0.4 m or greater, about 0.5 m or greater,or even larger. This relatively large size distinguishes many previouselectrodes that can only be produced in a small size, for example up tothe order of 0.01 m in size. The difference in size scale is not atrivial factor since many small electrodes cannot be simply scaled up insize. For example, in relatively small cells having small sizedelectrodes, it is not required to have or consider a high electricalconductivity in the cell/electrode, because the distances involved aresmall, so the associated resistances are relatively small. In contrast,in larger scale cells/electrodes, such as the present example, thisissue is much more challenging and higher conductivity is required alongvery good conduction pathways. Hence, a small scale electrode structurecannot typically and simply be scaled up to a large scale electrode.

Further aspects and details of example electrodes that can be utilisedas GDEs to improve or enhance managing electrochemical reactions can befound in the Applicant's concurrently filed PCT patent applications“Composite Three-Dimensional Electrodes and Methods of Fabrication”filed on 30 Jul. 2014, “Modular Electrochemical Cells” filed on 30 Jul.2014, and “Electro-synthetic or Electro-energy Cell with Gas DiffusionElectrode(s)” filed on 30 Jul. 2014, which are all incorporated hereinby reference.

It is to be understood that these examples are illustrative only andthat there are other ways in which the method and/or electrochemicalcell promote improved or optimum reaction conditions within a variety oftypes of GDEs having wetting pressures and/or bubbles points above about0.2 bar.

Example 5 Managing Electrochemical Reactions by Employing GDEs withWetting Pressures/Bubble Points Above 0.2 Bar

In one example there is provided a method for managing anelectrochemical reaction in an electrochemical cell, which in use has agas diffusion electrode positioned between a liquid electrolyte and agas region. The method includes the steps of selecting the gas diffusionelectrode to have a wetting pressure or a bubble point greater than 0.2bar, and applying a pressure differential between the liquid electrolyteand the gas region that is less than the wetting pressure (in the casewhere the liquid pressure is greater than the gas pressure) or thebubble point (in the case where the gas pressure is higher than theliquid pressure).

In another example there is provided an electrochemical cell whichincludes a gas diffusion electrode having a wetting pressure or a bubblepoint greater than 0.2 bar, and the gas diffusion electrode positionedbetween a liquid electrolyte and a gas region. In use a pressuredifferential is applied between the liquid electrolyte and the gasregion that is less than the wetting pressure (in the case where theliquid pressure is greater than the gas pressure) or the bubble point(in the case where the gas pressure is higher than the liquid pressure).

In another example, there is provided a method for managing anelectrochemical reaction in an electrochemical cell. The electrochemicalcell has a gas diffusion electrode positioned between a liquidelectrolyte and a gas region. The method comprises applying a pressuredifferential between the liquid electrolyte and the gas region that isless than a wetting pressure (in the case where the liquid pressure isgreater than the gas pressure) or the bubble point (in the case wherethe gas pressure is higher than the liquid pressure) of the gasdiffusion electrode relative to the liquid electrolyte and a gas used orconsumed in operation of the cell. The wetting pressure or the bubblepoint is greater than 0.2 bar.

A three-phase solid-liquid-gas interface is formed within the gasdiffusion electrode. In one example, an excess pressure on a liquidelectrolyte side of the gas diffusion electrode over a gas side of thegas diffusion electrode is less than the wetting pressure during use. Inanother example, an excess pressure on a gas side of the gas diffusionelectrode over a liquid electrolyte side of the gas diffusion electrodeis less than the bubble point during use. Preferably, the gas diffusionelectrode is bubble-free or substantially free of bubble formationduring use.

In another example, the gas diffusion electrode has a wetting pressureor a bubble point greater than or equal to 1 bar, and the pressuredifferential is less than 1 bar. In another example, the gas diffusionelectrode has a wetting pressure or a bubble point greater than or equalto 2 bar, and the pressure differential is less than 2 bar. In anotherexample, the gas diffusion electrode has a wetting pressure or a bubblepoint greater than or equal to 3 bar, and the pressure differential isless than 3 bar. In another example, the gas diffusion electrode has awetting pressure or a bubble point greater than or equal to 4 bar, andthe pressure differential is less than 4 bar. In another example, thegas diffusion electrode has a wetting pressure or a bubble point greaterthan or equal to 5 bar, and the pressure differential is less than 5bar. In another example, the gas diffusion electrode has a wettingpressure or a bubble point greater than or equal to 6 bar, and thepressure differential is less than 6 bar.

In various other examples, the pressure differential is set to about 0.1bar, about 0.2 bar or about 0.3 bar below the wetting pressure of thegas diffusion electrode when the liquid electrolyte has a higherpressure than the gas region. In various other examples, the pressuredifferential is set to about 0.1 bar, about 0.2 bar or about 0.3 barbelow the bubble point of the gas diffusion electrode when the gasregion has a higher pressure than the liquid electrolyte. In variousother examples, the pressure differential is set to between about 1 barto about 2 bar below the wetting pressure of the gas diffusion electrodewhen the liquid electrolyte has a higher pressure than the gas region.In various other examples, the pressure differential is set to betweenabout 1 bar to about 2 bar below the bubble point of the gas diffusionelectrode when the gas region has a higher pressure than the liquidelectrolyte.

In another example there is provided a pressure measurement device, suchas a pressure transducer, flow meter, pressure gauge, to measure thepressure differential. Also there can be provided a control deviceconfigured to adjust the pressure differential based on the measuredpressure differential. For example the control device could be aprocessor, computing device or unit, digital or analog electronic deviceor circuit, integrated circuit, software or firmware, that controlspressure or flow changing or altering mechanisms, such as a valve. Thecontrol device could adjust or maintain the pressure differential to bea preselected value, for example as input or set by a user using a userinterface or control panel. The control device could be configured tomaintain the pressure differential to be less than the wetting pressureor the bubble point. The control device could be configured to adjustthe pressure of the liquid electrolyte and/or configured to adjust thepressure in the gas region.

In yet another example, the gas diffusion electrode comprises: anon-conductive gas permeable material that is substantially impermeableto the liquid electrolyte, during normal operational use of theelectrode; and a porous conductive material provided on a liquidelectrolyte side of the gas diffusion electrode. The non-conductive gaspermeable material is provided on a gas side of the gas diffusionelectrode. The porous conductive material may be attached to thenon-conductive gas permeable material by being laminated to thenon-conductive gas permeable material. Alternatively, the porousconductive material may be attached to the non-conductive gas permeablematerial by being coated on at least part of the non-conductive gaspermeable material.

In other examples, the gas permeable material has a characteristic poresize less than about 250 nm; or the gas permeable material has anaverage pore size of between about 100 nm to about 500 nm, or in morespecific examples about 0.1, about 0.2, about 0.3, about 0.4 or about0.5 micron. Preferably, the gas permeable material has a pore size thatis substantially uniform. Substantially uniform pore size is intended tomean where less than 10% of the gas flow occurs through pores that areabout 50 times or more larger than the average pore size.

Example 6 Managing an Electrochemical Reaction for the Example of aWater Electrolysis Reaction

In this example an electrochemical reaction was managed by increasingthe excess (differential) pressure of the liquid side over the gas side,up to and beyond the wetting pressure of the GDE, for the representativecase of a water electrolysis reaction. To examine the different factorsinvolved in maximising GDE performance, several studies were carried outusing the example cell 600 depicted in FIGS. 5 and 6, equipped withidentically sized anode and cathode GDEs of 4 cm² area, which had beenfabricated by depositing a porous metallic element 20 (in this example100 line-per-inch stainless steel mesh, coated with and laminated usinga nickel nanoparticle catalyst containing 5% binder material being aporous polymer, such as a fluoropolymer-copolymer (e.g. Nafion™)) and agas permeable, electrolyte-impermeable layer (a PTFE membrane fromGeneral Electric Company for membrane distillation, having average poresize 0.2 μm, a wetting pressure of 3.4 bar and a bubble point of 2 bar),as shown in FIG. 3.

Cell 600 includes first side section 610, middle section 620 and secondside section 630, for example made of metal such as stainless steel,which can be bolted together. First gas regulator 640 transfers gasinto/from an electrolyte chamber of the cell. Second gas regulator 650transfers gas into/from a gas chamber of the cell. First electricalconnection 660 attaches to one electrode and second electricalconnection 670 attaches to another electrode, where one or both of theelectrodes is a GDE.

The cell was then charged with an electrolyte of 6 M KOH and cellvoltages of between 1.7 V and 2.5 V were applied over the electrodes,whilst applying pressures of up to 3 bar (=300 kPa) to the centralliquid electrolyte chamber via regulator 640. The rate of theelectrolysis reaction was measured by recording the current under theapplied conditions.

When equivalent solid-state electrodes were used in the cell instead ofthe above GDEs at atmospheric pressure, the recorded currents were ofthe order of 0.17 mA/cm² (at 1.7 V) to 2 mA/cm² (at 2.5 V).

FIG. 7 shows the obtained data using the above GDEs. As can be seen, thecurrents were substantially higher with the GDEs than with theequivalent conventional electrodes. Moreover, at all recorded voltagesthe current increased as the excess (differential) pressure of theliquid phase over the gas phase was increased. For example, at an equal,atmospheric pressure on both of the liquid and gas chambers(Pressure=0), a current of 133 mA/cm² was recorded at 2.5 V. When theliquid phase was then pressurised to 1 bar (=100 kPa) whilst maintainingthe gas phase at atmospheric pressure, then the current rose to 134mA/cm². At 2 bar (=200 kPa) for the liquid phase and atmosphericpressure for the gas phase, the current rose to 135 mA/cm². At 3 bar(=300 kPa) for the liquid phase and atmospheric pressure for the gasphase, the current rose to 136 mA/cm². Similar increases were seen forall of the different voltages applied.

This continued up to about 3 bar, whereafter at 3.4 bar and higher,liquid electrolyte penetrated and flooded the GDEs, completely stoppingthe reaction and yielding a zero current (as the gas-liquid interfacewas destroyed). Electrolyte also leaked into the gas chambers. Thisoccurred because the wetting pressure of the GDE was 3.4 bar.

When the pressure differential was taken back below 3 bar, then theflooding stopped and the reaction current slowly re-established itself(as the three-phase solid-liquid-gas interface was re-established).

This example shows that the use of the GDEs improved the rate of thereaction and, by implication, the efficiency of the reaction. Moreover,the application of an increasing differential pressure to the liquidover the gas side of the GDEs, resulted in a progressively furtherincrease in the rate of reaction. This continued up to the point wherethe applied differential pressure on the liquid side versus the gas sideexceeded the wetting pressure of the GDEs, which resulted in flooding ofthe GDEs, destroying the gas-liquid interface and halting the reaction.A quick decrease in the applied differential pressure to back below 3.4bar, re-established the gas-liquid interface, and re-started thereaction.

Example 7 Managing an Electrochemical Reaction for the Example of aWater Electrolysis Reaction

In this example an electrochemical reaction was managed by increasingthe absolute pressure of the liquid and gas sides of a GDE to 6.5 bar,whilst maintaining a differential pressure of the liquid side over thegas side of 0.5 bar, for the representative case of a water electrolysisreaction. The above experiment in Example 6 was repeated in a somewhatdifferent way using a slightly modified GDE for the anode. The onlydifference was that the catalyst-binder combination in the anode GDE wasaugmented by additional 50% CosO₄. Co₃O₄ is a catalyst of waterelectrolysis.

For this example, a constant excess (differential) pressure on theliquid over the gas side of the GDEs of 0.5 bar (=50 kPa) was maintainedat all times. The pressure on the gas and liquid sides were thensimultaneously increased in steps of 1 bar each.

FIG. 8 shows the resulting data. The pyramidal trace on the far left ofFIG. 8 was obtained at the different voltages shown when the pressureapplied to the liquid side was 0.5 bar and the pressure applied to thegas side was 0 bar. The designation at the top of each trace shows thepressure applied to the gas side.

The next pyramidal trace to the right (—that is, the second pyramidaltrace from the left) was taken when the pressure applied to the liquidside was 1.5 bar and the pressure applied to the gas side was 1 bar(marked on the Figure as “1 bar”).

The next pyramidal trace to the right was taken when the pressureapplied to the liquid side was 2.5 bar and the pressure applied to thegas side was 2 bar (marked on the Figure as “2 bar”).

The next pyramidal trace to the right was taken when the pressureapplied to the liquid side was 3.5 bar and the pressure applied to thegas side was 3 bar (marked on the Figure as “3 bar”).

The next pyramidal trace to the right was taken when the pressureapplied to the liquid side was 4.5 bar and the pressure applied to thegas side was 4 bar (marked on the Figure as “4 bar”).

The next pyramidal trace to the right was taken when the pressureapplied to the liquid side was 5.5 bar and the pressure applied to thegas side was 5 bar (marked on the Figure as “5 bar”).

The right-most pyramidal trace was taken when the pressure applied tothe liquid side was 6.5 bar and the pressure applied to the gas side was6 bar (unmarked on the Figure).

As can be seen, the reaction rate, as measured by the current (leftaxis; in mA) increased as the absolute pressure on the electrochemicalcell increased. Moreover, there was no flooding or even chance offlooding, since the differential pressure of the liquid over the gasside in all cases was only 0.5 bar, whereas the wetting pressure of theexample GDE used was 3.4 bar. Thus, a very large margin of error existedto cater for unexpected and sudden pressure swings.

This example shows that the use of the GDEs improved the rate of thereaction and, by implication, the efficiency of the reaction. Moreover,the application of an increasing absolute pressure to theelectrochemical cell, whilst maintaining an invariant excess (different)pressure for the liquid over the gas side of the GDEs, resulted in astill further increased rate of reaction. This continued up to at least6 bar of applied pressure on the gas and 6.5 bar of applied pressure onthe liquid electrolyte.

Example 8 Managing an Electrochemical Reaction for the Example of aWater Electrolysis Reaction

In this example the electrochemical reaction was managed by increasingthe absolute pressure of the liquid and gas sides of a GDE to 9 bar,whilst maintaining a differential pressure of the liquid side over thegas side of 1 bar, for the representative case of a water electrolysisreaction. The above experiment was repeated in a somewhat differentformat. The only difference was that 200 lines per inch nickel meshwithout any catalysts, was used for the conductor-catalyst layers inboth GDEs. Also a constant excess (differential) pressure of liquid overgas, of 1 bar (=100 kPa) was maintained throughout.

FIG. 9 shows a 3D graph of the data that resulted when the absolutepressure of the electrochemical cell was increased up to 9 bar (=900kPa) for the liquid and 8 bar (=800 kPa) for the gas side of the GDEs.

As can be seen, there was a moderate increase with increases in theabsolute pressure and the applied voltage.

This example shows that the use of the GDEs improved the rate of thereaction and, by implication, the efficiency of the reaction. Moreover,the application of an increasing absolute pressure to theelectrochemical cell, whilst maintaining an invariant excess (different)pressure for the liquid over the gas side of the GDEs, resulted in astill further increased rate of reaction. This continued up to at least8 bar of applied pressure on the gas and 9 bar of applied pressure onthe liquid electrolyte.

Example 9 Using Flexible 3D Electrodes to Form a Spiral-Wound Cell orDevice

As previously discussed, example 3D electrodes and GDEs can be flexible.The 3D electrodes or GDEs can be formed as anodes and cathodes for usein a variety of cells, devices or reactors. The 3D electrodes or GDEscan be stacked or layered, for example as alternating anodes/cathodesand with any required intervening spacer layers, insulating layers, gaschannel layers, feed channels or the like. Selected edges of the 3Delectrodes or GDEs can be sealed while other selected edges are leftunsealed for gas or liquid ingress or egress, as required.

FIG. 10 schematically illustrates an example partially producedspiral-wound cell, device or reactor 400. One or more flexible 3Delectrodes or GDEs 410, for example a layered stack of flexible 3Delectrodes or GDEs formed as anode-cathode pairs or series, can berolled or spiral-wound about a central tube, conduit or section 420.Some applications may call for a single flexible 3D electrode or GDE tobe rolled or wound.

FIG. 11 schematically illustrates an example of how flexible 3Delectrodes or GDEs, for example after being stacked as anode-cathodepairs or series, can be formed into an example spiral-wound cell, deviceor reactor 450. To minimise the overall footprint of a cell, amulti-layered arrangement of flat-sheet flexible 3D electrodes may berolled up into a spiral-wound cell 450. The spiral-wound cell 450 maythen be encased in a casing, which still allows for electrolyte totransit through the cell 450. 3D electrodes or GDEs acting as anodes andcathodes can be attached to a central tube 420 in such a way thatunsealed edges of the electrodes properly transport liquid/gases. Forexample, electrolyte can be introduced to the rolled 3D electrodes orGDEs at input edges 490, and electrolyte can exit the rolled 3Delectrodes or GDEs at exit edges 480. Also for example, a gas or gasescan be introduced to the rolled 3D electrodes or GDEs at gas input 460,and a gas or gases can exit the rolled 3D electrodes or GDEs at gas exit470. The liquid and gas plumbing can vary depending on the specificstructure or application. The liquid electrolyte and/or gas can bepressurized, or a pressure differential otherwise applied between theliquid electrolyte and the gas, as previously discussed.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

Optional embodiments may also be said to broadly consist in the parts,elements and features referred to or indicated herein, individually orcollectively, in any or all combinations of two or more of the parts,elements or features, and wherein specific integers are mentioned hereinwhich have known equivalents in the art to which the invention relates,such known equivalents are deemed to be incorporated herein as ifindividually set forth.

Although a preferred embodiment has been described in detail, it shouldbe understood that many modifications, changes, substitutions oralterations will be apparent to those skilled in the art withoutdeparting from the scope of the present invention.

1. A method for managing an electrochemical reaction in anelectrochemical cell, the electrochemical cell having a gas diffusionelectrode positioned between a liquid electrolyte and a gas region, themethod comprising: applying a pressure differential between the liquidelectrolyte and the gas region that is less than a wetting pressure or abubble point of the gas diffusion electrode relative to the liquidelectrolyte and a gas used or consumed in operation of the cell; whereinthe wetting pressure or the bubble point is greater than 0.2 bar.
 2. Themethod of claim 1, wherein a solid-liquid-gas interface is formed withinthe gas diffusion electrode.
 3. The method of claim 1, wherein an excesspressure on a liquid electrolyte side of the gas diffusion electrodeover a gas side of the gas diffusion electrode is less than the wettingpressure during use.
 4. The method of claim 1, wherein an excesspressure on a gas side of the gas diffusion electrode over a liquidelectrolyte side of the gas diffusion electrode is less than the bubblepoint during use.
 5. The method of claim 1, wherein the gas diffusionelectrode is bubble-free or substantially free of bubble formationduring use.
 6. The method of claim 1, wherein the wetting pressure orthe bubble point is greater than or equal to 1 bar, and the pressuredifferential is less than 1 bar.
 7. The method of claim 1, wherein thewetting pressure or the bubble point is greater than or equal to 2 bar,and the pressure differential is less than 2 bar.
 8. The method of claim1, wherein the wetting pressure or the bubble point is greater than orequal to 3 bar, and the pressure differential is less than 3 bar.
 9. Themethod of claim 1, wherein the wetting pressure or the bubble point isgreater than or equal to 4 bar, and the pressure differential is lessthan 4 bar.
 10. The method of claim 1, wherein the wetting pressure orthe bubble point is greater than or equal to 5 bar, and the pressuredifferential is less than 5 bar.
 11. The method of claim 1, wherein thewetting pressure or the bubble point is greater than or equal to 6 bar,and the pressure differential is less than 6 bar.
 12. The method ofclaim 1, wherein the pressure differential is set to about 0.1, about0.2 or about 0.3 bar below the wetting pressure of the gas diffusionelectrode when the liquid electrolyte has a higher pressure than the gasregion.
 13. The method of claim 1, wherein the pressure differential isset to about 0.1, about 0.2 or about 0.3 bar below the bubble point ofthe gas diffusion electrode when the gas region has a higher pressurethan the liquid electrolyte.
 14. The method of claim 1, wherein thepressure differential is set to between about 1 bar to about 2 bar belowthe wetting pressure of the gas diffusion electrode when the liquidelectrolyte has a higher pressure than the gas region.
 15. The method ofclaim 1, wherein the pressure differential is set to between about 1 barto about 2 bar below the bubble point of the gas diffusion electrodewhen the gas region has a higher pressure than the liquid electrolyte.16. An electrochemical cell comprising: a gas diffusion electrode havinga wetting pressure or a bubble point greater than 0.2 bar relative to aliquid electrolyte and a gas used or consumed in operation of the cell;and the gas diffusion electrode positioned between the liquidelectrolyte and a gas region; wherein in use a pressure differential isapplied between the liquid electrolyte and the gas region that is lessthan the wetting pressure or the bubble point.
 17. The cell of claim 16,wherein the gas diffusion electrode comprises: a non-conductive gaspermeable material that is substantially impermeable to the liquidelectrolyte during normal operational use of the electrode; and a porousconductive material provided on a liquid electrolyte side of the gasdiffusion electrode.
 18. The cell of claim 17, wherein thenon-conductive gas permeable material is provided on a gas side of thegas diffusion electrode.
 19. The cell of claim 17, wherein the porousconductive material is attached to the non-conductive gas permeablematerial by being laminated to the non-conductive gas permeablematerial.
 20. The cell of claim 17, wherein the porous conductivematerial is attached to the gas permeable material using a bindermaterial.
 21. The cell of claim 17, wherein the porous conductivematerial is coated on at least part of the non-conductive gas permeablematerial.
 22. The cell of claim 16, wherein the pressure differentialincreases the efficiency of the cell.
 23. The cell of claim 16, whereinbubbles of gas are not produced at the gas diffusion electrode.
 24. Thecell of claim 16, wherein the gas diffusion electrode includes a barrierlayer.
 25. The cell of claim 17, wherein the gas permeable material hasa characteristic pore size of about 50 nm to about 500 nm and is formedof PTFE.
 26. (canceled)
 27. (canceled)
 28. (canceled)
 29. The cell claim16, including a pressure measurement device to measure the pressuredifferential.
 30. The cell of claim 29, including a control deviceconfigured to adjust the pressure differential based on a measuredpressure differential.
 31. The cell of claim 30, wherein the controldevice adjusts the pressure differential to be a preselected value. 32.The cell of claim 30, wherein the control device is configured tomaintain the pressure differential to be less than the wetting pressureor the bubble point.
 33. The cell claim 30, wherein the control deviceis configured to adjust the pressure of the liquid electrolyte and/orthe pressure in the gas region.
 34. (canceled)